# PHP Fatal error: Uncaught Error: Class "X" not found

**Code:** `Class not found` · **Category:** PHP · **Seen on:** PHP

PHP couldn’t load the class — autoloading failed, the namespace is wrong, or the dependency isn’t installed.

## Meaning

Modern PHP relies on Composer’s PSR-4 autoloader. The class name (with namespace) must map to a file path that exists with matching case.

## Common causes

- `vendor/` missing — `composer install` not run on server
- Namespace/`use` statement wrong
- File name or directory case differs from class (works on macOS, fails on Linux)
- Autoload map stale after adding classes
- Missing PHP extension providing the class (e.g. `PDO`, `ZipArchive`)

## Quick fix

1. Run `composer install` (and `composer dump-autoload`)
2. Check `use` statements and namespace declarations
3. Match file name case to class name exactly
4. Enable the extension for built-in classes

## Detailed fix by platform

### PHP

1. `composer dump-autoload -o` after adding classes; verify `autoload.psr-4` in composer.json maps the namespace prefix to the right folder.

## Code examples

### PSR-4 mapping must match folders and case

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

With this mapping, `App\Services\Mailer` must live in `src/Services/Mailer.php` — exact case on Linux. Run `composer dump-autoload` after changes.

### Missing use statement

```php
namespace App\Http\Controllers;

use App\Models\Invoice;          // without this line PHP looks for App\Http\Controllers\Invoice

class BillingController {
    public function show(int $id) { return Invoice::find($id); }
}
```

## How to diagnose

1. **Name** — Fully qualified class name correct?
2. **Autoload** — Does PSR-4 map it to an existing file?
3. **Deploy** — Is vendor/ present on the server?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/php/class-not-found)