Alas, I'd love to have 1 asset that I could use for all conditions but it's just not available in the world of computing. You're going to have to learn 2 or more.
PHP is very widely used, so you might as well stick with it. If you can create decent webapps using it, go for it. I would suggest learning C/C++ too so you can write any high-performance modules using that and call them from your PHP code. That's probably the best of all worlds for your webapps.
If you wanted to write for desktops, I think you'll be best off learning C++ with Qt (and look at Wt) (as it appears you're a Linux dev), or C#/VB.NET for Windows.
For mobiles, learn C/C++ as you can write apps in that no matter which platform even if you have to put up with some platform-dependant extensions - you either have to learn Java for Android, Objective-C for iOS, or (well we're not quite sure what MS has planned for Windows Phone 8, but I hear they like native code again, that means C++/CX). You can see where I'm going with this!
so anyway, if you're happy with PHP then keep with it. There is a ton of code out there that runs PHP so it's not like you're working with some bleeding-edge or hardly-used obscure language.