This might just be me but I wouldnt neccessarilly right this script in PHP. PHP is a wonderful language it fast, and most of all opensource 🙂 . However, because it is not compiled (until runtime I think ??!?) and does not have the ability for multiprocesses, I dont think I would use PHP. I would probably use something heavy like, C or something. Unless you can find just some really cool trick. 🙂 But that is just me.
(Or you could get 120 computers all to share the same database right the script in PHP and have each computer send 10 emails every 5 seconds. :-) )
Hope that helps!