Jackon Miller looks (very briefly) at one point of difference between Ruby and PHP - the installation.

Steps for installing Ruby

1. ./configure
2. make
3. make install

He is being a little tongue-in-cheek here, but it is an interesting point. Sure, it's nice to have it where PHP is flexible and you can compile in whatever you'd like, but what about those that just want one or two things? Or maybe more, but don't want to go through the whole install process?