Moodle

Open Course E Learning/LMS.

http://en.wikipedia.org/wiki/Moodle

The stated philosophy of Moodle includes a Constructivist and Social Constructionist approach to education, emphasizing that learners (and not just teachers) can contribute to the educational experience in many ways. Moodle's features reflect this in various design aspects, such as making it possible for students to comment on entries in a database (or even to contribute entries themselves), or to work collaboratively in a wiki. Having said this, Moodle is flexible enough to allow for a full range of modes of teaching. It can be used for both introductory and advanced delivery of content (e.g. HTML pages) or assessment, and does not necessitate a constructivist teaching approach.

built in PHP

not a total solution http://zaidlearn.blogspot.com/2009/11/moodle-is-airport-not-total-solution.html