I'm an ex-PHP programmer (well, I still have to use some PHP at work, but I don't tell anyone that at parties) who now uses Python or Lisp wherever possible for web development. I can tell you exactly why PHP is so popular: it acts as an extension of HTML and is syntactically similar to Perl.
PHP mixes with HTML and removes the separation of model and view. It has its roots in strictly procedural code (and its OO colors were obviously painted on later), which is simple for non-programmers or beginning programmers to understand. When considered from the standpoint of a template language, it's a wonderful language. When considered as a real programming environment, its limitations show. PHP began as a series of Perl scripts. If you have ever looked at a huge PHP project, it looks just as unreadable as a huge Perl project. But there is a large number of programmers who began writing Perl- based CGI for the web, and PHP was a very natural next step for them. This is also a big reason why Ruby is popular among the same crowd (...I say as I duck the inevitable flame war I've just started). Ruby takes a lot of concepts from Perl and tries to make them work in an OO framework (the ~ operator and the built in regex type, for instance). I started with PHP for both of those reasons. Perl was becoming too much of a chore and PHP offered a solution without learning a new templating language: a way to embed perl code in the page itself. Of course, now I am in recovery and am proceeding well with my 12 steps, thanks for asking. -- http://mail.python.org/mailman/listinfo/python-list
