PSR-4 Autoload Namespace Mismatch

Why This Happens

PSR-4 autoloading maps namespaces to directories. If composer.json maps App\ to src/, then App\Controller\HomeController must be in src/Controller/HomeController.php with the namespace App\Controller. A mismatch anywhere in this chain causes the class to not be found.

The Problem

// composer.json: "App\\" => "src/"
// File: src/Controller/HomeController.php
namespace Controllers; // Wrong namespace!

class HomeController {}

The Fix

// composer.json: "App\\" => "src/"
// File: src/Controller/HomeController.php
namespace App\Controller; // Matches PSR-4 mapping

class HomeController {}

Step-by-Step Fix

1. 1

   Check composer.json autoload

   Look at the autoload.psr-4 section in composer.json to see how namespaces map to directories.

2. 2

   Verify the namespace

   Ensure the namespace in the PHP file matches what PSR-4 expects: base namespace + subdirectory path.

3. 3

   Rebuild autoloader

   Run composer dump-autoload -o to regenerate the optimized autoloader after fixing the namespace.