Fatal error: Uncaught Error: Class 'PHPUnit\Framework\TestCase' not found in C:\xampp\htdocs\amik7d\vendor\sebastian\environment\tests\OperatingSystemTest.php:17 Stack trace: #0 {main} thrown in C:\xampp\htdocs\amik7d\vendor\sebastian\environment\tests\OperatingSystemTest.php on line 17