Automatically add +1 every 30mins

[Alex Major]

Hi there. 
I've posted this up on both this list, and the php 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 mysql which would achieve this.

Regards, 
Alex.