Four years ago, I didn't know PHP but decided to use it on our new version of flag-ship product - it promised cross-platform compatability etc and er low-cost to own er free. Within a week I was cruising and displaying concept applications for the marketing department to comment. I'm no genius, PHP is just another choice albeit a good choice for me to move to. IMHO (without going in a head on debate on which scripting language is best for the web) if you once :
1) coded scripts such as PERL, VB
2) coded C++ or Delphi (for basic understanding of Objects)
If you are pretty good with any of the above then migration to PHP is as tough as erm... a good reference book to bring to the toilet once in a while! 🙂 E.g. you will only need to learn new syntax, all the old school programing still structures apply.
For me, PHP was easy - took the jump, and got a promotion thrown in for good measure. I'm sure there are horror stories out there, just that I haven't heard one yet! 🙂