My guess is that many developers probably don't actually choose PHP per se but rather get stuck using it because that's what the shop they work for uses, or that's what the current site is implemented in and they're only adding a feature, or that's what the client is asking for, etc.

More interesting to me is the question: Why is PHP beating Python in this space? Some guesses:

* PHP webapps far easier to deploy.

* PHP docs give more usage examples, and also allow user comments.