In a new post to the SearchCo.de blog Ben Boyter generated a listing of the most commonly used PHP functions/structures based on the contents of several of the major PHP projects from around the web.
One thing that I considered some time ago was working out which are the most common functions in a language and adding this as an additional signal to ranking. I couldn't find anywhere else on the web with this question answered so I took my own approch. The method was to take a collection of large PHP projects, including, Wordpress, Mambo, Sphider, Smarty, Drupal, CodeIgniter, dump all their source code into a single file stripped of comments, and then run some simple regex over this file counting the occurance of each function.
His results show the top five as: array, isset, define, empty and assert. The last five ended up being: filemtime, sha1, array_unshift, get_current_user and strchr.