PHP is ubiquitous. It is a powerful open source, server-side, interpreted language. It is quick to deploy, easy to attach to dozens of database models and it is fast. Its user-base is incredibly large and loyal and although it has its detractors - mostly Python users, Ruby users ... well, users from every other language ever created, it has managed to command a position in programming history that few could match.
What? PHP is probably the most loved and loathed language in the history of computing. It is also probably the most used and probably has the biggest user-base.
Why? Every man and his cat uses it. If you are good at PHP, you will probably stay employed for a long time.
Would you? Of course. I have earned a living from it for the last 8+years. I love it and don't see it ever waning in popularity.... apart from when Ruby takes over the world in a few years!
There are plenty of docs available online, so I will be just presenting some hints and tips I have picked up over the years.