I'll let you in on a little secret: nobody memorizes the syntax for every function - there are way too many of them in PHP. That's why I have the PHP manual bookmarked and use an editor with a PHP plug-in that provides context-sensitive function help and its own local copy of the manual. 🙂
As far as some sort of PHP exercises site, I don't know of any off-hand. I'd suggest thinking up some fairly minor functionality you'd like to add to a personal web site, perhaps, and then start looking for info on how to do it, writing the code, testing it, and asking for opinions and help here when you get stuck.
One thing I would suggest if you are new to object-oriented programming is Matt Zandstra's introduction to objects in PHP to get a leg up on the terminology and basic concepts. (A lot of PHP code out there is not object-oriented, but I suggest getting into it as soon as you can, instead of learning how to do everything in a procedural manner and then deciding to learn it all again in an OOP way.)