I would go with php because:
* it's far more widely used than ruby which means there is a lot more documentation on the net and there is also a lot more software already written in php which you can use / cannibalize.
* php is totally ubiquitous and installed on every host everywhere.
* php has its flaws but it's used robustly on some of the highest traffic sites on the net (Facebook, all the big porn tubes sites etc).
* ruby documentation is pretty scattered and thin from what I can tell.
If you want to lean a more fully featured scripting language than php I would go with python, which is also a nice clean language, well documented, and it has a great standard library out of the box.
If you want to learn PHP, I would highly, highly recommend this course:
Code:
http://web.archive.org/web/20100621105325/http://www.scriptschool.com/php/
It's very straight to the point and teaches you everything you need to know to get started making useful scripts. It's how I first learned!