Javascript would be the next language up I'd presume, I went on to AJAX which I'm learning slowly now.
You definitely need MySQL if you don't know that already. PHP and MySQL go together in so many ways, get on MySQL and then develop personal applications for yourself or friends in your free time. That's what I've been doing for the past few months and I've gotten much better then I thought I'd be now.
Whenever something came up that could have been computer related I thought of ways I could develop an application for it. For instance I wanted to budget my money a little bit so instead of using Excel, Mint, Quicken or some other program I developed my own application. I needed a news system and wanted others to be able to comment on my news and downloads as well as rank them so in my free time I started developing applications for that.