as i understand it, the purpose of mirroring is to take the load off of one server - ie you can get to a mirror of the php manual without overloading the php.net server. Hence, a mirror is some other site hosing a mirror of the information of a different site.
So if you want to mirror php.net, then you just get a copy of everything on their server and put it on yours.
It's not looking at the information of php.net but using a different url, it's actually hosting the information somewhere else so that the load is decreased from the php.net server.
