note 102318 modified in function.mt-rand by danbrown

This will result in an EXTREMELY fast random string using the following characters: "0123456789abcdefghijklmnopqrstuvwxyz"

you can use:

<?php
base_convert(mt_rand(0x1D39D3E06400000, 0x41C21CB8E0FFFFFF), 10, 36);
?>

This will be a random number/string always 12 characters. This will only work on 64 bit systems. This will result in a random number/string between: 100000000000 to zzzzzzzzzzzz If you are using a 32 bit system you will need to use:

<?php
base_convert(mt_rand(0x1679616, 0x39AA3FF, 10, 36);
?>
This will result in a 5 character random number/string, between: 10000 to zzzzz

--was--
Correction to what I said before about the fast random number/serial. This will result in an EXTREMELY fast random string using the following characters: "0123456789abcdefghijklmnopqrstuvwxyz"

you can use:

<?php
base_convert(mt_rand(0x1D39D3E06400000, 0x41C21CB8E0FFFFFF), 10, 36);
?>

This will be a random number/string always 12 characters. This will only work on 64 bit systems. This will result in a random number/string between: 100000000000 to zzzzzzzzzzzz If you are using a 32 bit system you will need to use:

<?php
base_convert(mt_rand(0x1679616, 0x39AA3FF, 10, 36);
?>
This will result in a 5 character random number/string, between: 10000 to zzzzz

http://php.net/manual/en/function.mt-rand.php