Generate unique login token

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