Using PHP With LDAP. One of the reasons for PHP's immense popularity is its support for a wide variety of different Internet technologies. It is this support, coupled with the speed with which new language extensions get added to the source tree, that keeps PHP ahead of other competing languages in the same category, and ensures the continuity (and future growth) of the fan following it currently enjoys.
One such Internet technology is LDAP, the Lightweight Directory Access Protocol. In case you're not familiar with LDAP, it is a protocol designed to allow quick, efficient searches of directory services. Built around Internet technologies, LDAP makes it possible to easily update and query directory services over standard TCP/IP connections, and includes a host of powerful features, including security, access control, data replication and support for Unicode. — Harish Kamath.
Part 1 and Part 2. [PHP Everywhere]