I really don't know how to advise you.
I know people who make $150 per hour coding c++ as senior programmers. I also know people who provide entire web site solutions for $10,000.00 - $15,000.00 per time and people who still code in lisp getting $100.00 per hour.
At the other end of the spectrum I know people giving their skills away free in a desperate attempt to try and gain extra experience.
I know someone who codes HTML for $8.00 per hour.
I think the guy above has the right idea....try to find out what they might be willing to pay..perhaps you could ask just how involved the PHP they are looking for will be...MYSQL interoperability and HTML echoing for exmple is a lot easier than shortest path algorithms for example.
Perhaps you could charge them per job....