Composer and autoload using your own classes

I haven't yet found a topline explainer of how the autoload function works in Composer, if you want to add a custom folder to composer's autoload function.

We're going to pretend we're using composer to autoload our project and run phpunit. My project will be called MyProject. Here is my class:

<?php
namespace MyProject;

class MyClass {

}
?>

Firstly, let's add composer to this project:

composer require --dev phpunit/phpunit

We also want Composer to autoload our MyProject class and to map the namespace to the correct folder in our project (e.g. src/).

After we added phpunit as above you will then find a file called composer.json in the top level of your project. Open it up and add an autoload section, with a nested psr-4 object.

{
    "require-dev": {
        "phpunit/phpunit": "^12.1"
    },
    "autoload": { 
        "psr-4": {

        }
    }
}

Inside the psr-4 object, we're going to create a key/value pair. This key-value pair maps the namespace declared in your class to a real folder in your project:

{"namespace\\" : "path/to/class"};

For example:

{
    "require-dev": {
        "phpunit/phpunit": "^12.1"
    },
    "autoload": { 
        "psr-4": {
            "MyProject\\" : "src/MyProject/classes/"

        }
    }
}

Once Done, you need to dump the autoload folder, so composer can relink your namespace to the class folder:

composer dump-autoload

Enjoy!

Thomas - 27-05-2025