One final vote for C, coming from somebody's who's been along a similar learning path. I gleaned most of my programming (at least the modern useful stuff) from PHP, but started getting into doing embedded device development and found that PHP had all the capabilities, but was WAY too slow and resource-intensive. Started looking into C, and found it amazingly easy to learn because the syntax and concepts are almost exactly the same. (to the point where half of my PHP code could have almost been copied and pasted, compiled and it ran in C sort of thing). The real trick to learning C from a PHP point of view is just to get the whole memory management thing figured out, which really isn't as hard as people make it sound... you just need to make sure you plan and pay a lot of attention when you create variables that you remember to free them later, that's all. If you start getting into UIs and things it can be a little more complex, but for back-end/daemon type stuff it's not hard at all.