Wouldn't you simply look for any include(), include_once(), require() or require_once() statements in your document?
There is a thread floating around (that dates back to 2002 if I'm not mistaken) regarding which editor people are using.. there are some PHP editors that might have some additional info somewhere in their GUIs which keeps track of variable used, funtions used, etc.. perhaps also listing any included files are utilised in the document. But I could be wrong on this count. Mabey others can confirm / deny this. But at the top of my head, this is all that comes to mind.
Cheers,
NRG
*Edit: Here is the thread:
http://www.phpbuilder.com/board/showthread.php?t=10209798