PHP is a great language to know. If nothing else, it is a great starting point.
It's simpler to learn than most languages. You can kind of start of on simpler terms.
Many of the other languages are completely OOP, and while PHP does support OOP, you don't have to use it, or even know what it is, unless you want to.
Wether it is worth investing a lot of time in, career vise. My guess is that demand for PHP developers is only going to go up. The fact that it's open-source has scared away a lot of enterprise level user, but I think that's becoming less of an issue.