Why not continue with PHP?
Become an expert and then a guru in it. PHP is a pretty large subject and in several years of development I didn't get to use and know all it's aspects, no matter how many bigger or smaller projects I did. There are also different approaches in PHP coding, nowadays the frameworks emerged and they are a topic by themselves.
And not the last thing, probably because of its popularity, PHP is a living language and continue to evolves. New stuff appears on daily bases, modules, new versions, new applications, new frameworks, new problems to solve with it.
