I would actually say that the developer who has experience in a handful of languages/framework and is willing to learn php for the sake of a cool project/team/c

I would actually say that the developer who has experience in a handful of languages/framework and is willing to learn php for the sake of a cool project/team/company is potentially a better developer than one that has just mastered php (note: not saying php dev == bad, but that multi-langage/framework dev that values problems over tools more typically == good). So by limiting your offering to just good php devs you may actually be driving away some of the best devs interested in working for you.