The two most common powerful editors for Unix, emacs and vim, both include the ability to reference a list of PHP function names so that you can jump to their definition while editing. You'll have to build the file yourself first, though, using ctags. The best implementation I know of is Exuberant Ctags by Darren Hiebert, which is available in most Linux distributions. Normally this is called just by running ctags -R, but by default ctags assumes that all PHP code is in files with names ending in .php.