On a side note, I will use PHP on my servers before touching Ruby since I see no advantages for using it over PHP.
Except for where speed is of the utmost importance (in which case you're limited to C/C++, and possibly just assembly language), anything you can do in one language, you can pretty much do in any language. The question is, how EASY is it to do it in a certain language.
There may be no web app you can create in Ruby that you can't also create in PHP, but that doesn't mean PHP is always equally suitable to the job at hand. You may find that Ruby allows you to complete certain projects much faster, saving you a LOT of money. And in those cases, that would be a big advantage for using it over PHP.
I tell them to turn to the study of mathematics, for it is only there that they might escape the lusts of the flesh. -- Thomas Mann, "The Magic Mountain"