[PHP] Generate unique login token

[sean greenslade]

I have a login system that I am coding. I need it to generate a unique token
on login to be stored in the browser's cookie. I currently use a script that
generates a MD5 hash of the current unix timestamp, then checks the mysql
database to see if the token already exists. It loops this generate/check
until it gets a unique token. Is there a better way to do this? I want a
token that cannot be easiy predicted (i.e. not an auto-increment value).
Thanks in advance!
-- 
--Zootboy