The vast majority of Bitcoin users are technically-savvy, therefore you need to provide a transparent algorithm, which will ensure fairness even without trusting to the gambling site.
+1, and this is exactly what I meant to be provably fair. Using the PHP random function is "fair", but not "provably fair", who can assure that you really used the php function to select the winner?
your site is very similar to another bitcoin lottery that's already known in the bitcoin community, so, what exaclty your site have to offer beyond what is already being offered in the other site that will bring players to your site?
regards