How to test PHP code a user submits through a textarea

I'm trying to write a couple of simple PHP questions that people can solve. For example, a questions could be to write a little script that sorts a given array. Another questions could be to write a piece of code that creates a certain object with a couple of methods.

I then want to validate the answers a user submits through a textarea, along the times of what codecademy is doing.

What would be the preferred way to validate PHP code that was submitted in such a way?