PHP or Ruby

I know both have their advantages...

Starting to sharpen my programming skills, need to know by your opinion, which would you rather be an expert on?

The goal is to have technical expertise to build really useful sites for things widely used like social media sites, shopping carts, integrated payment processing, advanced user registration and interaction, etc..

The main question is how much do you want to pay for infrastructure. PHP infrastructure is widespread. Ruby doesn’t scale near as well and will require a lot more resources for a much smaller return.