How PHP and Composer find your code

Recently I showed how to create an empty Packagist package. By “empty”, I mean it doesn’t contain any code. This is useless, so let’s start adding some code.

The package is jameshfisher/donut-logic, which is on at The package is backed by a Git repository at To add the package as a dependency of your project, run

$ ./composer.phar require jameshfisher/donut-logic

This adds jameshfisher/donut-logic to your composer.json, and puts the contents of the package under vendor/jameshfisher/donut-logic/. I want to add some code to this package so that, from my project, I can use it as follows:

echo(\Jim\DonutLogic\DonutAdder::addDonuts(2, 3));  // prints "O"

In the above, \Jim\DonutLogic is a namespace name, DonutAdder is a class name, and addDonuts is a static method on that class.

When your code makes this call, how does PHP find the implementation of \Jim\DonutLogic\DonutAdder::addDonuts? The standard answer is: you must have defined it before you call it. You can define addDonuts in the lines above the method call, but this can’t work for things defined in other packages. Instead, you use include/require/require_once, which, given a filename, load the file as PHP. Just ensure that you’ve included the file which defines \Jim\DonutLogic\DonutAdder::addDonuts before you call it. Using Composer, you include the file by including your project’s vendor/autoload.php:

require __DIR__ . '/vendor/autoload.php';
echo(\Jim\DonutLogic\DonutAdder::addDonuts(2, 3));  // prints "O"

The autoload.php could work by includeing all of your dependencies’ source files. However, this is not how modern PHP works! Instead, modern PHP uses a PHP feature called “autoloading”. At runtime, when PHP encounters the use of a class which has not been defined, it delegates the loading to an “autoloader”. An autoloader is a standard PHP function which is passed the name of a class, and is expected to somehow define that class. You can register your autoloader with spl_autoload_register($my_autoloader_func). There can be many autoloaders.

When our PHP requests to use \Jim\DonutLogic\DonutAdder, PHP realizes it’s not defined, and asks the autoloaders to define it. There are lots of ways the autoloaders could do this, which would be chaos. To avoid such chaos, the PHP community have defined PSR-4, which says how an autoloader should find a file based on a class name. One implementation of PSR-4 is Composer’s autoloader, and that’s what vendor/autoload.php defines.

PSR-4 says that the class DonutAdder must be in a file DonutAdder.php. We’ll put the source file DonutAdder.php in our package source repository at src/DonutAdder.php, so that Composer will install it at vendor/jameshfisher/donut-logic/src/DonutAdder.php. Then we must arrange for Composer’s autoloader to find the file at that location. PSR-4 says that the file will be found in a directory determined by two things: the namespace (\Jim\DonutLogic), and a mapping from namespace prefixes to “base directories”. In our case, the namespace prefix \Jim\DonutLogic must be mapped to the directory vendor/jameshfisher/donut-logic/src.

Composer’s prefix-to-directory mapping is defined by dependencies’ composer.json files. I must extend the donut-logic package’s composer.json as follows:

    "name": "jameshfisher/donut-logic",
    "description": "Shared logic related to donuts",
    "require": {},
    "autoload": {
        "psr-4": {"Jim\\DonutLogic\\": "src/"}

In src/DonutAdder.php, we define the \Jim\DonutLogic\DonutAdder class:

namespace Jim\DonutLogic;
class DonutAdder {
  public static function addDonuts($a, $b) {
    return "O";

In my example project, after a ./composer.phar update, the following code works:

require __DIR__ . '/vendor/autoload.php';
echo(\Jim\DonutLogic\DonutAdder::addDonuts(2, 3));  // prints "O"

I wrote this because I felt like it. This post is not associated with my employer.