Automatically +1 every 30 mins to a value in the database.

Hi there. I've posted this up on both this list, and the mysql list as I'm not sure whether this is something that I'd need to do with the php or mysql. Basically, I am making an add-on to my small website which is a mini online game. Every user will have gold, and every 30mins I'd like their amount of gold to go up by 1 (or say a variable say $goldupdateamount). I'd like to know which would be the best way of doing this, and if there is a command in php which would achieve this. Regards, Alex.