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.
An Ada exception is when a routine gets in trouble and says 'Beam me up, Scotty'.