For anyone who has experience with PHP and LDAP:

One of the ideas behind LDAP is that it is fast. Does this hold true for the PHP LDAP implementation and is it faster than using MySQL for user data?

At extremely high loads, yes, it's probably faster than MySQL. But it also is more complicated, which introduces more potential security issues/risks. Also there are a lot of tasks that it just isn't capable of carrying out... especially in terms of interfacing with daemons and such.

Depending on what you plan to do, it may be a way to go. But it's far from an do-all end-all system.

Oh, and on the PHP side of it. LDAP works quite well, PHP has extensive support for it, far superior to most other languages.