[PHP-WIN] Random Code

[Sigurd Magnusson]

Whats the best way to make a completely random, unique code in the format "ABCD-1234" 
where each of the 8 characters can be alphanumerc; 0-9,A-Z... I thought about 
generating a 36^4 number, converting to to base '36', and repeating the process. 
What's a good way to ensure the code is not repeated, though?

Siggy