Generate random key

[Mythotical]

I haven't figured out yet how to generate a random key that consists of letters and numbers. I managed to get it to generate numbers but for the life of me I can't seem to figure out how to generate a random alphanumeric key.

Here is what I am using for numeric only:

PHP Code:

mt_rand(); 


I know, thats simple, but I am using it temporarily till I figure out how to generate a key that consists of alphanumeric and is length is 16, no matter how many key's it generates.

Any help greatly appreciated

EDIT: Also, I have read quite a few articles on php.net but alas have come up empty handed.

Thanks in advance
Steve M