Did you try sourceforge yet?
Search for "Schedule" (~500 results) then choose "Filter" and adjust the form to say
require | programming language | PHP (93 results).
Make a shortlist of about between 3-10 which look hopeful, check the activity on the account, the last edits, if they have a site, look at the licence etc, and if cool - download them and see if/how they match your requirements.
On the face of it your requirements are pretty simple, am pretty sure we could help you with stuff.
Try and absolutely nail the spec before you go too much further, did you create a simple wireframe of the screens with the DMUs (Decision Making Units) yet? Have you spoken to one or two of the real users? [their POVs may not match].
God, 4000 posts - a serious addiction. (thats not counting the old SP personas I dumped)