[rdrimmie]

I think python, but of the three I only have first-hand experience in it and php. I think it depends on what languages you currently know, and which of them is the most different.

I started learning PHP in 2001, coming from web development in ASP3 written in VBScript. At the time, developing pages in PHP and VBScript was very similar, and it was really more a matter of learning the few syntax differences and the core library.

Python is a much different language than PHP and VBScript, and as a result working on learning it has made me a much better programmer and it has been a lot more interesting because I'm working with concepts that I might not otherwise cross paths with in my day-to-day programming and gives me a leg up as these concepts are added to php.

If you haven't already, please also consider learning javascript. There's a lot of nascent node.js webframeworks that you could start with.