PSR-4 Autoloading Basics

In the last lesson you wrote an autoloader that maps App\Service\Greeter to src/Service/Greeter.php. If every project invented its own mapping, sharing code would be a mess. PSR-4 is the agreed standard.

PSR stands for PHP Standards Recommendation. They come from the PHP-FIG group. PSR-4 describes how a fully qualified class name maps to a file path.

The rule

1. A namespace prefix is mapped to a base folder.
2. The rest of the class name is turned into a path: each \ becomes /, and .php is added.

What you must do

• One class (or interface, trait, enum) per file.
• The file name equals the type name exactly, including capitals.
• The namespace in the file matches the folder path.

A file src/Service/Greeter.php must declare:

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<?php

namespace App\Service;

class Greeter {}

A tiny PSR-4 autoloader

This shows how a PSR-4 mapper works.

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<?php
function psr4Path(string $class, string $prefix, string $baseDir): ?string
{
    if (!str_starts_with($class, $prefix)) {
        return null;
    }

    $relative = substr($class, strlen($prefix));

    return rtrim($baseDir, "/") . "/" . str_replace("\\", "/", $relative) . ".php";
}

echo psr4Path("App\\Service\\Greeter", "App\\", "src") . "\n";
var_dump(psr4Path("Vendor\\Thing", "App\\", "src"));

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src/Service/Greeter.php
NULL

Several mappings

A project can map several prefixes:

Why this standard matters

• Every Composer package follows it, so installing a package “just works”.
• Any tool or editor can find the file for a class name.
• You can move between projects and know where things are.

Telling Composer

You do not write the autoloader. You describe the mapping in composer.json, and Composer generates the code.

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{
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    }
}

The next chapter covers Composer, including this setting in PSR-4 Autoloading with Composer.

Other PSRs you will hear about

You do not need to memorise them. They show up when you use libraries and frameworks.