I'll also chime in with some personal experience. The only formal education I've had in programming has been in C/C++, but I picked up PHP even before that just by using the manual, a few tutorials here and there, and a lot of "reverse engineering" of sorts by looking at the source of already-made PHP applications and understanding how they work. That, and as leatherback suggests, starting out with small (and progressively growing), well-defined tasks/projects that might not have been very practical to actually use but definitely helped further my knowledge/experience with PHP.
In addition to the resources leatherback listed, I would also suggest taking advantage of the underused Code Critique forum at any and all stages of your learning with PHP. It's a great place to show us what you came up with and see how others might have done it differently or improve upon it.
