How to Fix "Fatal Error: Class Not Found" in PHP: Composer PSR-4 Autoloading Guide
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Publisher ID: ca-pub-XXXXXXXXXXXXXXXX (Configured in config.php)The fatal error "Class 'App\Models\User' not found" indicates the Composer autoloader or spl_autoload_register failed to locate and require the corresponding PHP file. The most common cause is a discrepancy between folder names and namespace capitalization.
1) Run "composer dump-autoload -o" in your terminal to regenerate class maps. 2) Ensure your namespace in composer.json matches the folder structure exactly (e.g. "App\\": "src/"). 3) Check character case: Windows is case-insensitive, but Linux servers strictly enforce case (e.g. UserController.php vs usercontroller.php).
📋 Guide Table of Contents
1. Problem Diagnosis & Symptoms
Code that runs seamlessly on local Windows development machines frequently crashes upon deployment to Linux shared hosting due to filename case differences.
2. Primary Root Causes
- Cause: Linux case-sensitivity: filename is User.php but imported as user.php.
- Cause: Forgot to run composer dump-autoload after creating new classes.
3. Step-by-Step Diagnostic Fixes
Regenerate the Optimized Autoloader
Execute Composer optimization command:
composer dump-autoload -o4. How to Verify the Resolution
- Instantiate the class and verify class_exists($class) returns true.
5. Diagnostic Troubleshooting Matrix
| Symptom Observed | Root Cause | Recommended Action |
|---|---|---|
| Works on Windows localhost, fails on Hostinger Linux | Filename or folder casing mismatch with namespace | Ensure directory and file casing matches namespace declaration exactly |
6. Critical Mistakes to Avoid
- ❌ Manually writing require_once for every file instead of using Composer PSR-4.
7. Frequently Asked Questions
❓ Where should require "vendor/autoload.php" be placed?
At the very top of your application entry point (index.php).
Official Technical References
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