PHP vs Ruby
Which is better?
If the question is php vs rails. You aren't comparing apples to apples. Rails is a framework and php is a language. If you are gonna use PHP without a framework, you might consider using Rails to get your project done faster. This is assuming you have no experience with either language.
I would argue for Ruby, despite its drawbacks. It will teach you to be a better programmer IMO. You might also consider Python, which I love and adore.
Since you are doing the work for someone else, you have to ask yourself about the relationship you plan to have with these people.
If you select PHP, they'll have millions of developers to pick from. If you select Ruby, they'll find it that much harder to replace you.