查找包含类定义的PHP脚本的路径/名称

I'am looking for a way to find out, witch php script contain a class definition, because i got an error:

"Cannot redeclare class MyClassName"

I only have one script in my project, that contain the class definition and require it this way:

require_once(dirname(__FILE__).'/path/to/MyClassName.php');

When i use class_exists('MyClassName') right befor the require_once, it return true, that means the function has defined.

So now i would like to find out, what script contains the definition.

Is the a function to return a scriptname by given Classname?