Debugging open source code

I downloaded some open source project they used php language to develop i am using

netbeans ide for editing the php when i want to edit the source i will open the file and

edit it its working fine now i want to trace the code debugging option is not showing for

this files but when i create some file and try to debug it its working what is the problem

i'm not getting and i'm new to this.

can any1 help me on this