Have you looked into using pre-existing cross-reference tools?

Searching Google for "xref for PHP" I got these hits that look interesting:

• http://phpxref.sourceforge.net/
• http://xref-lint.net/

I then searched for "PHP cross reference" to turn up these additional ones:

• http://phpcrossref.com/
• http://bertgarcia.com/php-cross-reference-library

It strikes me, though, that you may want a more general tool. OpenGrok comes to mind. So does LXR.