With that in mind, may I make a suggestion re: your PHP position? I totally understand the below-market pay you are offering (35-50 USD/hr); after all, you are a non-profit with a limited budget, and there are other rewards for working for a non-profit. Nonetheless, it will likely prove very difficult to find a qualified developer at those rates, particularly once willing to work with PHP. And yet, you are requiring 5 years of development experience. Do you really need someone with that much experience?
If I were you, I would consider lowering that requirement to 2 years at the most. There are probably some well-qualified PHP devs just starting out who might like to work for you, and who would be perfectly capable of meeting your requirements (btw, this does not include me, since I haven't done significant work in PHP since 2003).
Finally, if you choose not to lower those requirements, you might want to ask yourself why a developer with 5 years of experience is willing to work for pay so far below the going rate. Unless he/she is a philanthropist whose life mission is to spread the use of audio books, I'd be willing to bet that this applicant is a very poor developer. Wouldn't you rather have a talented, eager developer with a year or less of experience?
Normally, I wouldn't take the time to write this kind of missive, but I appreciate your work and ideals and just want to alert you to going labor market conditions.
tl;dr: it's a hot job market for developers, so you might want to re-consider your hiring requirements.