Homebrew seems to be a really powerful package manager for osx, and the amount of available formulas is really convenient, but I can't understand something : Why isn't there any PHP formula for homebrew ?

I mean, there are "custom formulas" to add to homebrew, but no official one. 
The weird part is there are a lot of PHP extension available in hombrew, out of the box, such as apc, imagick, xdebug, ffmpeg-php but no PHP. I can't understand. Could someone explain ? Is there a reason ? A licence issue, maybe ?