I actually learned Rails after knowing PHP, and I felt that my knowledge of web fundamentals (generally, how web sites are built and architected) was greatly enhanced by my PHP knowledge. It also gives you an appreciation of what Rails makes easy!

If you're familiar with a C-based programming language, then PHP will be very fast to pick up. And, if you want to "actually be able to build something," with PHP you'll absolutely at least feel like you can haha.