I personally find that asking a coding test of junior programmers is a GREAT tool for helping to evaluate them. Typically just a very simple test, a basic CRUD application. Looking at what they create not only gives you insight into how they think and code (did they do MVC? a simple single .php file? handle web security? etc). It also gives you a great starting point to begin discussions with them in person, asking them about why they did (or didn’t) do certain things.
Those are things that a basic PHP programmer should at least know of.