If you are using codeigniter 2.0 and getting following error
Fatal error: Class ‘MY_Controller’ not found in applicationcontrollerswelcome.php on line 4
then reasons can be
1. you are either not loading it properly (via an __autoload() or manually using include())
2. it is in the wrong place.
In CI 2.0 MY_Controller should be in application/core. In previous version it should be in application/library