Just to say something that I haven't seen anyone else say, yet.
Have you worked with any PHP frameworks, like CakePHP or symfony? I know you said you've worked with PHP, so maybe you have. And you're probably a far more advanced PHP programmer than I am, because my experience is fairly limited, but I just started looking into these different frameworks and it's been a great experience for me. It facilitates design, but it's also given me some insight into MVC, granted it does so at an abstract level since it does a lot of the work for me, but its been a good experience for learning some new concepts.
Maybe you already have experience with frameworks like these, and this is not to say you shouldn't go and learn other languages and platforms, and indeed learning other languages is great and I'm sure people will respond with some ideas for that. But since you have experience with PHP, it might be worth looking into some different ways that you can use that knowledge and still gain some new insight into other technologies.