Unique ID - again

Hi folks, I'm using usual md5(microtime()); to create IDs, but now I've encountered a problem with that. I need to create explicitly 6 digit unique number(decimal). Yes, I know I can use for/while loop to fill a string with digits, but is it ABSOLUTELY sure that the random will never return same number when seed is microtime()? It is very important as the number will identify a bank transfer... Is there any other way than checking with all previous IDs? Or some MySQL function to do this for me? thank you Alex