I've had a photo challenge site called FujiMugs hosted here on lunarpages for a while now. It began as a site for photos taken with Fujifilm digital cameras, but an "open" category was added nearly a year ago for those with other camera models. The site is written using PHP to interface with MySQL databases - I've learned a lot about both of those doing all this.
The challenges repeat every few weeks. The winner of each challenge chooses the subject for the next challenge, then you have approximately two weeks to take and submit a photo that fits the subject before the 4 day voting period (photos must be taken after the subject is announced).
Check it out, sign up, and enter the challenges, vote, etc. (you do not have to enter a challenge in order to vote and/or leave comments on the entries). We'll be voting on challenge #48 this weekend, then challenge #49 will begin next week.http://www.fujimugs.com