PHP
Autoload (PSR-4)
Learn Composer's PSR-4 autoload and the use of vendor/autoload.php.
What is vendor/autoload.php?
After install, Composer produces a file named vendor/autoload.php. Calling it with require_once loads every class in the project automatically; you never write manual require or include.
The PSR-4 mapping in the autoload section binds class names to file paths. For example the App\ to src/ pair routes the App\Models\User class to the src/Models/User.php file.
composer dump-autoload
The composer dump-autoload command regenerates the vendor/autoload.php file. When you add a new class or change the autoload mapping, you run this command.
It also runs automatically after composer install or composer require; but when you only want to refresh the mapping, dump-autoload can be run on its own.
Exercise PSR-4 path resolver
Write a resolveClass(string $class, array $config): string function that mimics Composer's autoload psr-4 section. Iterate $config["autoload"]["psr-4"]; if $class starts with a prefix (check strpos against 0), strip the prefix with substr, turn the remaining \ into / with str_replace, prepend base and append .php, then return.
The $config array has autoload => psr-4 => ["App\\" => "src/"]. Print paths for App\Models\User and App\Services\Auth.
Show example solution
string { foreach ($config["autoload"]["psr-4"] as $prefix => $base) { if (strpos($class, $prefix) === 0) { $relative = substr($class, strlen($prefix)); $relative = str_replace("\\", "/", $relative); return $base . $relative . ".php"; } } return ""; }. Print with echo resolveClass("App\\Models\\User", $config) . "\n"; and echo resolveClass("App\\Services\\Auth", $config) . "\n";.
Example solution:
<?php
$config = ["autoload" => ["psr-4" => ["App\\" => "src/"]]];
function resolveClass(string $class, array $config): string {
foreach ($config["autoload"]["psr-4"] as $prefix => $base) {
if (strpos($class, $prefix) === 0) {
$relative = substr($class, strlen($prefix));
$relative = str_replace("\\", "/", $relative);
return $base . $relative . ".php";
}
}
return "";
}
echo resolveClass("App\\Models\\User", $config) . "\n";
echo resolveClass("App\\Services\\Auth", $config) . "\n";
Test yourself
-
Match the PSR-4 mappings
- App\Models\User
- src/Models/User.php
- App\Services\Auth
- src/Services/Auth.php
- App\Controller\Home
- src/Controller/Home.php
- App\Repository\UserRepo
- src/Repository/UserRepo.php
-
What does require_once 'vendor/autoload.php'; do?
- It installs a new package
- It checks version constraints
- It deletes the vendor/ folder
- It loads every class automatically
Show answer
It loads every class automatically — Correct. require_once vendor/autoload.php invokes the PSR-4 autoloader.
-
Complete the autoload call
<?php
// Load every class automatically
____ "vendor/autoload.php";
$app = new App\Application();
echo $app->name . "\n";
Show answer
<?php
// Load every class automatically
require_once "vendor/autoload.php";
$app = new App\Application();
echo $app->name . "\n";
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