Honestly, and this isn't because I'm just a fan of free bitcoins - some kind of giveaway or bounty usually attracts a lot of users. If you look at the view count for threads in this board that are prefixed with [BOUNTY], you'll notice it's often 10 or 20 times as high as the rest of the threads.

I guess a sig referral scheme would be the easiest to administrate, and you get free advertising from it. My only advice would be to make sure you'll definitely have the funds to pay out if it becomes popular; people get antsy if they're made to wait for payments!

edit also, it's great to see someone still using PHP - every day I feel like my skills are becoming more and more obsolete...