PHP and Doctrine experience are preferred.
However, though Tom may disagree with me (and, of course, he has the last word), I would personally say that all that should be required is solid programming skill, a willingness to learn (carrying with it a willingness to be, essentially, on probation for the first few months), and a good attitude about the whole thing.
When I started as a dev, I'd never touched a line of PHP. I learned it on the job, and I am fully confident that others could do the same, provided they have a good foundation.
It's not like we're oversupplied with PHP/Doctrine gurus volunteering, after all 
Hey if we are allowed to learn PHP/Doctrine on the job, I think I can sign up to help too. And wouldn't mind beeing on probation for a few months.
I´ll send Tom an email about it.
