How do we deal with dependencies in PHP

Generally what we want to do when we’re coding something is to call a function.

do_the_thing();

While some of the functions we call could stand alone and some could be methods on an object, they’re all just function calls.

So why does most PHP code have so many classes? I don’t think code organization is a good answer, since we have access to namespaces which can do that just as well. I believe the only valid reason apart from value objects is because of trying to manage dependencies.

Let’s say you are writing code which wants to refund a purchase through a billing system. You call something like refund_purchase($purchase) and you expect to get back a receipt id for the refund. From most caller’s perspectives, that’s all we should need to worry about.

However, let’s say the refund function needs to do the following things:

  • Contact the payment processor to initiate the refund.
  • Create and save a new receipt to the database.
  • Queue webhooks for the refund.

In order to do any of those things, it requires references to other systems. What if we want to switch out those other systems? Perhaps we want to test our function without a database, or we want to change the payment processor. This is where dependency injection comes into play.

Dependencies as Arguments

In order to use dependency injection in the simplest sense, we need to pass interfaces for each of the dependent systems into our function as arguments.

function refund_purchase($purchase, $processor, $database, $webhooks) {}

This is not scalable, though, because as the number of dependencies increases, so does the number of arguments, making it very confusing to call the function in the first place. In addition, when a developer wants to add a new refund call, they also have to figure out how to get each one of the dependencies, each of which may have their own dependencies!

Wrappers

The alternative to passing dependencies as arguments is to use a wrapper function to pre-inject our dependencies before they are needed. In PHP (and other Object-Oriented languages) the most accepted way to do this is using a class constructor as that wrapper. (In some other languages we might use a Higher Order Function, but this is much more challenging in PHP).

class Refunds {
  public function __construct($processor, $database, $webhooks) {
    $this->processor = $processor;
    $this->database = $database;
    $this->webhooks = $webhooks;
  }

  public function refund_purchase($purchase) {}
}

But now we are stuck with another problem: were do we call the wrapper? Since we might want to ask for a refund in any number of places, generally we must force the calling code to call the wrapper as well.

$wrapper = new Refunds($processor, $database, $webhooks);
$wrapper->refund_purchase($purchase);

That’s not a whole lot better than putting the dependencies into the function arguments, though; it’s confusing and annoying for the caller. Also, any time we want to add new dependencies we would have to change every single caller.

Factories

A solution to the issue of where to call the wrapper is to put it into a stand-alone function.

function create_refund() {
  global $wpdb;
  $processor = new DefaultProcessor();
  $database = $wpdb;
  $webhooks = new Webhooks();
  return new Refunds($processor, $database, $webhooks);
}

The calling code must still create the wrapper before it uses the function it wants, but at least that can be broken down to a one-liner.

create_refund()->refund_purchase($purchase);

In practice, these wrapper calls are often put into static functions, and can even be functions attached to the classes they are meant to wrap, which puts them close to the function call we want and so makes them easier to discover.

Forwarding Functions

It’s also possible to wrap the wrapper call into its own forwarding function to remove any need for the caller to know about our wrappers.

function refund_purchase_with_defaults($purchase) {
  return create_refund()->refund_purchase($purchase);
}

This may seem like an extreme level of redirection, but it provides the best of both worlds: as long as the forwarding function does not do anything apart from call the wrapper and its method, most callers can ignore dependency injection entirely. This only works if we make sure never to use the forwarding functions inside other wrappers, though.

Containers

Eventually we end up with tons of wrappers, as there are many functions that have dependencies and they can’t always be grouped easily. This presents two new problems: it can be hard to locate the wrapper you need, and in order to keep everything as simple as possible for the caller, we end up creating and re-creating the wrapper class many times within a single execution of the code. This can immensely bloat our memory usage since each one of those instances keeps its own state and references.

Therefore many projects end up with a Dependency Injection Container which puts all the wrapper functions into one place and also caches the instances it creates, making it very easy for callers to keep their one-liners without having to worry about memory issues.

class Container {
  public function create_refund() {
    global $wpdb;
    if (! $this->refunds) {
      $processor = new DefaultProcessor();
      $database = $wpdb;
      $webhooks = new Webhooks();
      $this->refunds = new Refunds($processor, $database, $webhooks);
    }
    return $this->refunds;
  }
}

Having these wrappers creates a new problem for our functions themselves, though, which I mentioned above: it’s now so easy to call other wrapped functions that it’s tempting to use the wrappers to make calls to other systems rather than using injected references. Doing this defeats the purpose of our wrappers entirely and we end up right back where we started. Therefore we must have some mechanism, usually just code review, which prevents anyone from using wrappers within a wrapped function.

Do we need all this?

All of this work is built on the premise that dependency injection is good and a necessary thing to have. The counter-argument is that without that premise, our code is so much simpler. So let’s examine the concept a bit more.

In any given project there will probably be functions for which the dependencies are relatively trivial. Let’s say a function needs to log events for analysis, but we don’t want to log anything while running automated tests. Rather than injecting a logger, we could just have the logger function itself detect if we’re running inside a test and if so, do nothing. If more special cases are needed, those can be added directly to the function as well.

function log_message($message) {
  if (! defined('ARE_TESTS_RUNNING')) {
    write_log_message($message);
  }
}

This technique has the advantage that it’s more foolproof than relying on callers to substitute dependencies, so we might be doing it anyway to protect our data. Even though it relies on global variables and constants, a PHP process is bounded in both time and scope so in many projects this reliance on globals creates fewer problems than it solves.

It’s also likely that in any given project there are at least a few functions which have very complex dependencies, and therefore are hard to test without running them in isolation. These functions probably benefit from using a wrapper approach as described above.

So when exactly do we need dependency injection and when is it just needless work? Even though to me the elegance of separating dependencies is a reason in itself, to be honest I can only think of two real needs. I think that we must ask these questions of each dependency we find.

Questions to ask

  1. Is a dependency large or complex enough to require mocking during testing, or do we need to test the interaction with a dependency itself?
  2. Is it likely that a dependency will need to be changed now or in the future because it represents some system that has more than one implementation?

As soon as any dependency in a project answers “yes” to one of the above questions, I think that it’s worth including a Dependency Injection system of some kind. Even if the answers are “no”, it might be a good idea if the project is a library which itself is used by other systems, because it’s often hard to predict how a library might be used. It can be a pain to be forced by your libraries to adopt specific other libraries.

And if you do have a Dependency Injection system, if any given dependency does not answer “yes” to the above questions, then perhaps you can ignore injecting it. The purist in me rebels against the idea of a single function including both injected dependencies and non-injected dependencies, but given the large amount of code needed to use a wrapper in PHP, it may just be the better choice.

(Photo by John Carlisle on Unsplash)

Safely coding with constants

In PHP there is a tendency to assume that the code we are working on is the only code that is running. The global and transactional nature of the language’s past has made this easy to do.

This tendency naturally leads us to use global state, and while there does seem to be a resistance to using global variables, I’ve only seen increased usage of constants created using define(). I would like to suggest that constants created in this way are actually just global variables by another name. Worse, perhaps, they are global variables whose values cannot be changed during runtime. If that last bit seems like an advantage rather than a disadvantage, read on.

First let’s briefly cover the risk of global state. As I’ve seen in my experience over and over again, the nature of code is to proliferate over time and when code starts to have complexity, global state becomes a liability. Anything the programmer cannot see directly when coding is something they have to keep in their head. And there is only so much we can keep in our heads, especially when large periods of time pass and when many different people are involved. A local variable (local state) is much easier to reason about, because it’s “right there” in front of us and less likely to be affected by things “somewhere else”.

There’s more to it than just making code more readable, though. Another way we learn about and protect code from bugs is by testing, both automated and manual. The nature of testing is to try some code, then change the state and try it again to see what the effects are. Remember how I said that constants created by define() cannot change? Let’s look at an example,

function getGroceries(): string {
  if (defined('LIKES_PEAS')) {
    return purchasePeas();
  }
  return purchaseCarrots();
}
// ...
echo "I will buy some " . getGroceries();

How would we write a test case for this function? The risk of having a condition on a defined constant is that apart from manual testing it is impossible to test it. Constants by their very nature cannot change, and defined constants are global in scope so there is nowhere they will not be. Once we define it, we cannot change it during the same runtime and most PHP test runners execute all their test cases in the same runtime.

That’s not to say that all constants are problematic. It’s more an issue of how we use them. Essentially, if we just stop writing if (defined())… in our code, the problem goes away.

But it’s not that easy, is it? The argument that I’ve heard most often to keep checking for defined constants is that they already exist in most large projects and so we have no choice. But this is not quite true.

The values stored in these constants are not themselves constant, and can be injected into the code that uses them. Here’s that previous example, written a different way,

function getGroceries(bool $likesPeas): string {
  if ($likesPeas) {
    return purchasePeas();
  }
  return purchaseCarrots();
}
// ...
echo "I will buy some " . getGroceries(defined('LIKES_PEAS'));

Suddenly it’s very easy to test the function! We just change its input. This is called dependency injection.

This technique is a time honored way to isolate dependence from logic. It may be less convenient because we must admit that when we need to check a constant we are actually adding a new dependency. However, noticing this gives us information, not penance. It may force us to find other ways of structuring our logic to avoid those dependencies altogether.

There is no doubt in my mind that the isolation of dependencies is critical to write code that is easy to reason about, and that doing so will reduce bugs, stress, and confusion as the code grows. When we recognize that constants created using define() are just another dependency, we can use them sparingly and isolate their usage in the same way we would for any other global or remote state which we cannot control.

(Photo by Simon Matzinger on Unsplash)

Creating Sniffs for a PHPCS Standard

As a follow-up to my summary of using phpcs to lint your PHP code, this post will explain how to create, modify, and test a phpcs standard.

As a quick refresher, phpcs organizes its linting messages (warnings and errors) into “sniffs”, then into “categories”, then into a “standard”.

The official tutorial for creating a standard is here, but it doesn’t go into much detail and some parts of it are out-of-date. There’s additional useful information in the 3.0 upgrade guide but I’m going to try to do a better job here of explaining how it all works.

Continue reading “Creating Sniffs for a PHPCS Standard”

Linting PHP with phpcs sniffs

Linting is the process of using an automated tool to scan your code for problems before you commit and deploy. It is a practice widely used in the development workflow of many languages, but hasn’t much been used in the PHP of WordPress developers.

The most commonly used linter in PHP right now is called “PHP Code Sniffer”, or phpcs for short. In this post I’ll show you how to install it in a project folder, configure it, and set up your editor to display its messages.

Sniff Codes

To phpcs, an “error” or a “warning” is generated by a function that checks PHP code for certain conditions.

phpcs calls a set of related errors or warnings a “sniff”.

Sniffs can be grouped together into a “category”.

phpcs calls a collection of categories a “standard”.

To make multiple sniff standards available in a single project, install the composer package dealerdirect/phpcodesniffer-composer-installer. That will automatically use all installed standards in the current project with the composer type phpcodesniffer-standard when you run phpcs.

To install that package in a PHP project, run the following (assuming you already have composer installed).

composer require --dev dealerdirect/phpcodesniffer-composer-installer

Each error or warning in a sniff has a “sniff code” which identifies it explicitly. A full sniff code includes the standard, the category, the sniff, and the error or warning, all separated by periods.

Standard.Category.Sniff.Error

For example:

MyStandard.MagicMethods.DisallowMagicGet.MagicGet

It’s possible to refer to all the errors in a sniff by omitting the error name. It’s further possible to refer to all the sniffs in a category, or the categories in a standard, but omitting those fields as well.

Therefore, these are all valid codes:

MyStandard.MagicMethods.DisallowMagicGet.MagicGet
MyStandard.MagicMethods.DisallowMagicGet
MyStandard.MagicMethods
MyStandard

Built-in Standards

phpcs comes with several standards built-in. You can see them on the command-line by running phpcs -i. These include PEAR, PSR1, PSR2, Squiz and Zend.

WordPress has its own standard which you can install with composer as well.

composer require --dev wp-coding-standards/wpcs

I highly recommend the VariableAnalysis standard which looks for undefined and unused variables.

composer require --dev sirbrillig/phpcs-variable-analysis

Configuring Standards

When installing sniff standards in a project, you edit a phpcs.xml file with the rule tag inside the ruleset tag. The ref attribute of that tag should specify a standard, category, sniff, or error code to enable. It’s also possible to use these tags to disable or modify certain rules. The official annotated file explains how to do this.

For example, this would enable all the sniffs in a standard called MyStandard:



  My PHP project.
  

The following configuration will enable all the sniffs in MyStandard, VariableAnalysis, and WordPress.



 My PHP project.
 
 
 

Configuring An Editor

To run phpcs on a file in your project, just use the command-line as follows (the -s causes the sniff code to be shown, which is very important for learning about an error).

vendor/bin/phpcs -s src/MyProject/MyClass.php

When it runs, phpcs will report any errors or warnings it finds. That’s pretty handy, but I find that it’s even more valuable to have linters run directly in your editor so that you can see problems as you code. Pretty much all editors support this, but the method of installation depends on your editor.

For vim, I recommend you install the ALE plugin. This has phpcs support already built-in and will use the standards you have configured in your project.

For phpstorm, the linting support is built-in, but you have to configure the app to know where to look. This guide shows how to set the path and enable the inspections. The guide neglects to mention that the path setting is under Preferences > Languages & Frameworks > PHP > Code Sniffer, and that the path should be set to your local installation of vendor/bin/phpcs. It also neglects to say that the inspections are under Preferences > Editor > Inspections and that you have to set the “Coding standard” to your phpcs.xml file manually (you may also want to disable some of the existing Inspections).

For vscode, I suggest installing the vscode-phpcs extension. It will automatically detect and use your local phpcs and configuration for your project. However, note that before version 1.0, the sniff code could not be shown.

Ignoring Rules

Sometimes you’ll need to ignore or disable one of the sniffs in a standard. The official documentation explains how to do this. You can surround or prefix your code with special comments to instruct phpcs to ignore it. Note that prior to version 3.2, you could not disable specific sniffs in this way; phpcs would ignore all of them for the marked lines.

$xmlPackage = new XMLPackage;
// @codingStandardsIgnoreStart
$xmlPackage['error_code'] = get_default_error_code_value();
$xmlPackage->send();
// @codingStandardsIgnoreEnd
$xmlPackage = new XMLPackage;
// @codingStandardsIgnoreLine
$xmlPackage['error_code'] = get_default_error_code_value();
$xmlPackage->send();

In the recent version 3.2 of phpcs, this feature was greatly improved. The above special comments were replaced with // phpcs:disable/// phpcs:enable and // phpcs:ignore respectively. You can also use // phpcs:ignore on the same line as a statement.

Even better, you are able to disable specific rules with either form by listing the sniff codes (comma-delimited) at the end of the special comment. For example, this following is possible.

doSomethingBad(); // phpcs:ignore MyStandard.Functions.PreventBadThings

You can even include comments in the disabling line!

doSomethingBad(); // phpcs:ignore MyStandard.Functions.PreventBadThings -- We are ok with bad things here

If there are specific sniffs you wish to ignore for your entire project, you can configure them in the phpcs.xml file and set their severity to 0. Here’s an example of disabling the WordPress function comment rule (standards can contain other standards, so some WordPress rules are actually part of the Squiz standard).



 My PHP project.
 
 
 
     0
 

In a later post, I explain how a phpcs standard is built and how to create and modify your own. Until then, see if you can find some standards that you like and use them! Here is the phpcs.xml file I typically use right now for non-WordPress projects.

(Photo by Noel Lopez on Unsplash)

Maybe returning errors in PHP

A common pattern I see in WordPress PHP code (and probably other PHP code) is a function which does some operation on data and returns a value. For example, a function which makes a database query and then returns the resulting row.

In order to make more robust code and prevent bugs, I’ve been systematically trying to use more return type declarations in my functions. This is a problem for things like the database function described above because a database query (and therefore the function itself) can sometimes fail. In those cases, the function might return the intended data, or it might return an instance of WP_Error instead. PHP does not currently (as of 7.1) support the idea of declaring return types when multiple types are possible.

Of course, such a function could simply throw an Exception if it fails, but this might be not as safe for critical code. An unhandled Exception, even a trivial one, could kill the whole program. Returning an error object instead could keep the program running (depending on how it is used). Even if we do throw an Exception, it’s not always clear when reading the function that it can throw.

So there are two problems now. First, since the function can return multiple things, we cannot specify any return type at all.

What I would need to solve that is the ability to specify multiple return types like phpdoc, but even if I could, it doesn’t guarantee that the user of this function will notice and handle potential errors.

This leaves us with the second problem, which is the same as if the function could throw an Exception; it can be easy for someone using it to overlook that the function can fail.

Is there a solution to both these problems? Maybe. (That’s a poor joke, as you’ll soon see.)

Let’s take the following function as an example. This function accepts an array of user data and returns the color property.

function getColorFromUser(array $user): string {
  return $user['color'];
}

It’s contrived, but it’s a reasonable simplification of the database query problem, since the array property might not exist, and therefore it can fail.

So now let’s modify the function to return an error if it fails.

function getColorFromUser(array $user) {
  if (! isset($user['color']) {
    return new WP_Error('NO_COLOR', 'Color does not exist');
  }
  return $user['color'];
}

As you can see, we’ve lost both the return type and also made it possible to introduce bugs in code that calls this function if they expect a string and instead get an object.

$myColor = getColorFromUser($user);
echo 'My favorite color is ' . $myColor; // This could cause a problem if $myColor is not a string

The standard way to handle this situation is similar to using a try/catch for an Exception; if we are aware that the function can return an error, we can check for one before continuing.

$myColor = getColorFromUser($user);
if (! is_wp_error($myColor)) {
  echo 'My favorite color is ' . $myColor;
}

It’s just that in my experience it’s common to miss the fact that a function can return an error. It’s even worse if a function once returned only a value but is subsequently changed to be able to return errors. Since the normal (successful) flow of the code will not change, no change is actually required anywhere the function is used. This makes type errors not only possible, but likely.

But what if we instead returned a wrapper object around the data? Let’s call it a Maybe. (See? There’s the joke.)

function getColorFromUser(array $user): Maybe {
  if (! isset($user['color']) {
    return Maybe::fromError(new WP_Error('NO_COLOR', 'Color does not exist'));
  }
  return Maybe::fromValue($user['color']);
}

Now we can have a return type, which can help prevent bugs inside the function. It’s not perfect as it doesn’t explain what types of values the object can contain, but it’s better than nothing. (To be fair, if we wished, we could create specific versions of Maybe for different values, like MaybeString, but I’m just going to keep it simple for now.)

Your first reaction might be that it also creates an inconvenience for the user of this function, because in order to extract the value of this data, someone has to do it explicitly.

But that small inconvenience means the developer must know about the possible error.

The example might then look something like the following. The condition required is similar to using is_wp_error(), but it’s more probable that the developer will remember to add the condition because they are forced to use getValue() on success.

$myColor = getColorFromUser($user);
if (! $myColor->isError()) {
  echo 'My favorite color is ' . $myColor->getValue();
}

This is not a perfect solution, and I’m certain that many developers will balk at having to add a pattern like this to their workflows when it’s not a standard part of the language. Personally I think it would be worth the discomfort to adopt this sort of construct when the benefit would be code that’s significantly less likely to have type errors. It probably depends on how careful you need your code to be. What do you think?

PS: It’s perhaps relevant to mention that there are functional programming concepts that are similar to this implementation, but which take it much further. My suggestion here is a very minimal interpretation that might be easier for some developers to grok. A more accurate name for this concept, matching existing patterns, is a Result.

Here’s how the Maybe/Result could be written:

(Photo by Caleb Jones on Unsplash)

Functional Dependency Injection in PHP

Having become used to the convenience of passing first-class functions around in JavaScript to make other functions decoupled and easily testable, I was wondering if we could do the same thing in PHP.

Of course, PHP technically has first-class functions as well, but the syntax is a little awkward, since the functions must be referenced as a string (and sometimes as an array).

Often I have some logic that can easily be contained in a pure function, and while I could put it in a class method, it’s not uncommon that the function doesn’t feel like it belongs to any particular class.

In these cases I tend to create “helper” classes. Essentially, I create a god class which is really just there to namespace its functions. If there’s a group of functions I want to inject, I might use regular class methods for this purpose, injecting an instance of the class. Otherwise, I use static methods. It occurred to me that maybe I could do the same thing with plain functions and actual namespaces.

JavaScript

In JavaScript one could do the following (this example is a bit contrived, but bear with me).

export function getGreenTea() {
    return 'green tea';
}

export function getOolongTea() {
    return 'oolong tea';
}
import {getGreenTea, getOolongTea} from 'tea-types';

function makeTea(getTea) {
    console.log(getTea());
}

makeTea(getGreenTea);
makeTea(getOolongTea);

PHP instance methods

Here’s how it could be done in PHP with classes and instance methods.

class GreenTea {
    public function getTea() {
        return 'green tea';
    }
}

class OolongTea {
    public function getTea() {
        return 'oolong tea';
    }
}

public function makeTea($teaType) {
    echo $teaType->getTea() . "\n";
}

makeTea(new GreenTea());
makeTea(new OolongTea());

PHP static methods

Here’s how it could be done with classes and static functions.

class TeaTypes {
    public static function getGreenTea() {
        return 'green tea';
    }

    public static function getOolongTea() {
        return 'oolong tea';
    }
}

function makeTea($getTea) {
    echo $getTea() . "\n";
}

makeTea([TeaTypes::class, 'getGreenTea']);
makeTea([TeaTypes::class, 'getOolongTea']);

PHP namespace functions

Lastly, here’s how it could be done with just namespaces. I think this is the closest to the JavaScript version, and doesn’t require creating unnecessary classes.

namespace TeaTypes;

function getGreenTea() {
    return 'green tea';
}

function getOolongTea() {
    return 'oolong tea';
}
namespace TeaMaker;

require('./TeaTypes.php');

function makeTea($getTea) {
    echo $getTea() . "\n";
}

makeTea('\TeaTypes\getGreenTea');
makeTea('\TeaTypes\getOolongTea');

Of course, this technique might not be the best choice for some situations. If there is any shared state or expensive repeated operations between functions, then they should be in a class and the methods should not be static.

Also, passing lots of dependencies to functions can sometimes be verbose and hard to read. In these cases it can be easier to pass the dependencies as a single object instance instead.

Finally, if there are several unrelated helper functions which are used in multiple places within the methods of a class, it can be more readable to pass all the helpers to the class constructor at once as an object (as though they were related).

Still, I think that passing functions as dependency injection is an under-used technique in PHP. I know it’s not the least bit Object-Oriented, and PHP is much more of an OOP language than JavaScript. In my opinion, however, the power of OOP is shared state and polymorphism and pure functions need neither.

(The image for this post is by 童 彤 on Unsplash.)

PHP Unit Testing and Pad Thai

As a follow-up to my last post about PHP unit testing, I recently gave a talk at WordCamp Boston about how to write testable WordPress plugins. You can see the talk slides here:

https://payton.codes/testable-wordpress-plugins/

You can also watch the talk here, although the audio quality isn’t great.

The premise of my talk was mainly that having testable code is a really great idea, even (and perhaps this is surprising) if there are no actual tests. Of course, without writing tests it’s hard to know for sure, but some experience will go a long way for future projects.

Even though this was at a WordCamp, my talk is not really WordPress specific. It really applies to any unit testing in PHP. In it, I gave an analogy to help my audience understand some of the concepts. Here’s that analogy:

Why have tests?

If you asked a friend to make you Pad Thai, you’ll probably get something edible, but maybe your friend isn’t such a good cook. If you don’t like the food, how can you tell what went wrong?

Dependencies

Defining your dependencies is important. “I want Pad Thai” vs. “I want Pad Thai, made with the following ingredients” will help. Even better, “I want Pad Thai, made with the following ingredients, ordered from this store, using this recipe”.

Mocks (AKA Stubs)

Using mocks is like asking the store to only sell your friend a particular brand of noodles when he calls. If the dish still tastes bad, you know it wasn’t because he got the wrong noodles.

What is a Spy?

Using spies is like giving your friend your phone number instead of the store’s phone number so that when he calls the store to order noodles, he calls you instead. That way you can verify that he’s ordering the right thing. If he calls and orders wheat noodles instead of rice noodles, you’ve found a bug with your friend’s recipe.

I also wrote Payton’s Testable Precepts: 10 guidelines I use when writing code that help make my code testable (as well as more clean, more flexible, and more readable). Here’s a list of those Precepts. See the slides for reasons and examples.

  1. Always use classes, never global functions.
  2. Don’t use static functions except as factories.
  3. Use instance variables as constants.
  4. Use filters to pass data indirectly.
  5. Use verbs in all function names.
  6. Keep functions below eight lines and indentation below four levels.
  7. Consider all possible inputs and outputs.
  8. Whenever possible, write tests first.
  9. Don’t test the code from other libraries, but test the inputs and outputs.
  10. Write only one assertion per test.