Required field only if another field has a value, must be empty otherwise

My question is about Laravel validation rules.

I have two inputs a and b. a is a select input with three possible values: x, y and z. I want to write this rule:

b must have a value only if a values is x. And b must be empty otherwise.

Is there a way to write such a rule? I tried required_with, required_without but it seems it can not cover my case.

In other words, if the previous explanation was not clear enough:

• If a == x, b must have a value.
• If a != x, b must be empty.

via Chebli Mohamed