How can anyone tell you how long it will take ou to master a technology?
Do research into php and you will see how easy or hard it is to pick up. You might be surprised why a bunch of people are moving towards that language.
The second question is somehow related to the first one in that AJAX relates mainly to the client side of the application and the supporting language for the server can be PHP, Java (Servelts/JSP), C# (ASP.NET) etc.. There are AJAX frameworks that have only client libraries (Dojo, Prototype etc) or the are AJAX frameworks dedicated to a specific language. Yout can find a comprehensive list of such frameworks at http://ajaxpatterns.org/wiki/index.php?title=AJAXFrameworks