PHP gets a lot of well-deserved flak for its relatively poor language design, but Jeff Atwood makes a good observation:
The only conclusion I can draw is that building a compelling application is far more important than choice of language. While PHP wouldn’t be my choice, and if pressed, I might argue that it should never be the choice for any rational human being sitting in front of a computer, I can’t argue with the results.
While PHP may be a poor choice for new applications, there are some amazing applications written in PHP. Namely, the Drupal CMS/Framework is an excellent example of a mature application written in PHP. If Drupal, or a Drupal clone were to be written in another language, I would switch in a heartbeat. Don’t get me wrong, there are CMS applications for Python and Ruby, but they require a great deal of development just to get the basic features that Drupal has now.
Umbraco (.NET, mostly C#) is another semi-mature CMS. It’s not as full-featured as Drupal, but its XML-driven content system is promising.
Python and Ruby developers: If you build it, they will come.