Get to know the PHP Manual, it is truly your best friend. I downloaded a copy for use on my development laptop.
As for programming platforms, I assume you are talking about frameworks. Some popular ones are CakePHP, CodeIgniter, Zend and Symfony.
As to which is best, I suggest you try them yourself. When I started PHP back in 2000, there was no framework, so I kind of created one that suits me and still sticks with it. For today PHP world though, you may be better off using one of the framework I mentioned above.
A lot of people seems to use Code Igniter, so you may try that first. You can also check with your employer to see if they use any framework and use that as a starting point. As you progress, you can always try the other frameworks.