random letter generator

[Butthead]

I'm creating an eCard system, and one of the things I was doing was generating a random 8 digit number for the pickup ID. Well, I'm not so sure I want to do it that way anymore. What about a 16 digit random letter(lowercase/capital) and number generator? How would something like this be done for extra security?

random numbers, then md5 it?

ideas?

note:I'm not looking for free code, just tips and ideas.

[mrhappiness]

echo uniqid(md5(microtime()));

why not use something like this?