Unique Identifier String

I am currently working on a mySQL site administration system and I need help on figuring out how I can create a 24 digit letter/number unique identifier string. I know that I can easily get a random number using a PHP function (I can remember it off the top of my head) but I also need to create random letters and then insert them into the number. Here are the steps of the process: 1. Create a 12 digit random number using a PHP function 2. Create 12 random letters 3. Insert the letters into the number, or the other way around Result: 1h5l79G05OP65kN61LPs1o6Z Out of the steps above, I am not sure how to do 2 and 3 and I would greatly appreciate it if someone could point out for me how to do them. Thanks in advance.