Adam Culp has posted his own look at some of the PHP usage statistics that are out there and how they can be interpreted.

Every once in awhile I stumble across someone who is trying to find their way and decide what they will do in their career. As the organizer of a PHP user group I see many new developers passing through. Of course I always speak of how strong PHP is in the web markets, and encourage new web developers to pursue PHP as a tool in their box of goodies. Because as a web developer it would be a career limiting move to not have any knowledge of PHP. Here is why...

He shares a few different sources including w3tech's overall and PHP-specific information (PHP5 specific here) and the current results of the TIOBE index showing language popularity. For each he talks some about what the results mean (and don't mean) and how, if you're a "professional developer" you should, at the least, know PHP - the most dominant language in the web space.