To start building a custom Composer plugin, first understand its basic structure and how it works. 1. Understand the basics of Composer plug-in: Plugin is a PHP package that implements Composer\Plugin\PluginInterface, which can hook internal events of Composer; key components include plug-in classes, event listeners and commands. 2. Set the plug-in structure: Create a project containing the src directory and composer.json, define PSR-4 automatic loading and specify the plug-in class. 3. Hook events or add commands: Register the event listener through the activate() method or implement CommandProviderInterface to introduce CLI commands. 4. Local test plug-in: Use the path repository to load the plug-in in the composer.json of the test project to ensure that the function is normal and then publish it to Packagist.

You want to build a custom Composer plugin, but don't know where to start — that's totally normal. The good news is: it's not as complicated as it might look at first. Composer plugins are basically PHP packages that hook into Composer's internal events and behaviors. If you've ever built a basic PHP package, you're already halfway there.

1. Understand the basics of Composer plugins

Before jumping into code, it helps to understand how Composer plugs work under the hood. A Composer plugin is just a regular Composer package that implements the Composer\Plugin\PluginInterface . This interface lets your code interact with Composer during various stages — like when installing packages or updating dependencies.

Key components:

• Plugin class : Your main entry point that implements the required interface.
• Event listeners : Optional handlers for Composer events (like post-install-cmd ).
• Commands : You can add custom CLI commands if needed.

The most important thing to remember: your plugin needs to be registered in the composer.json file of the project using it, and it must follow PSR-4 autoloading standards.

2. Set up your plugin structure

Start by creating a new directory for your plugin. Here's a minimum folder layout:

 my-composer-plugin/
├── src/
│ └── MyPlugin.php
├── composer.json

In MyPlugin.php , create a class that implements PluginInterface . It should look something like this:

 namespace My\Namespace;

use Composer\Plugin\PluginInterface;
use Composer\Composer;
use Composer\IO\IOInterface;

class MyPlugin implements PluginInterface
{
    public function activate(Composer $composer, IOInterface $io)
    {
        // This is where you register commands or event hooks
    }
}

Then, in your composer.json , define the namespace and point to your plugin class:

 {
    "name": "yourname/my-composer-plugin",
    "type": "composer-plugin",
    "autoload": {
        "psr-4": { "My\\Namespace\\": "src/" }
    },
    "extra": {
        "class": "My\\Namespace\\MyPlugin"
    }
}

This setup tells Composer where to find your plugin and how to load it.

3. Hook into Composer events or add custom commands

Most plugins either listen to Composer events or introduce new CLI commands. Let's say you want to do something after installation completes — you can attach an event listener.

Inside the activate() method:

 $dispatcher = $composer->getEventDispatcher();
$dispatcher->addListener('post-install-cmd', [$this, 'onPostInstall']);

And then define the handler:

 public function onPostInstall(Event $event)
{
    $event->getIO()->write("Thanks for running install!");
}

Alternatively, if you want to add a custom command like composer my-command , you'll need to implement CommandProviderInterface and return an array of custom commands.

This part gets more complex depending on what you're trying to do, but the official Composer documentation has solid examples.

4. Test your plugin locally before publishing

Testing a Composer plugin isn't always straightforward. One easy way is to use a local test project and require your plugin via a path repository.

In your test project's composer.json :

 {
    "repositories": [
        {
            "type": "path",
            "url": "../path-to-your-plugin"
        }
    ],
    "require": {
        "yourname/my-composer-plugin": "*"
    }
}

Now run composer install , and your plugin will be loaded and activated. This helps catch issues early without needing to publish anything.

Once you're confident it works, you can tag a version and push it to Packagist so others can install it normally.

That's basically it. Writing a Composer plugin doesn't have to be intimidating once you understand the structure and lifecycle. Just remember to keep things simple, test thoroughly, and document how to use your plugin.

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