phptagsfe - is a frontend for phptags writen in C using the gtk-2.0 toolkit. You might want to ask "what is phptags?". Here's the answer:

The phptags utility makes a tags file from the specified PHP sources. A tags file gives the locations of specified objects in a group of files. Each line of the tags file contains the object name, the file in which it is defined, and a search pattern for the object definition, separated by white-space. Using the tags file, many editors (ex(1), vim(1), emacs(1), etc) can quickly locate these object definitions.