I wrote a quick and dirty PHP class to convert an integer into a character string using a specific base and character set. If you wanted to use abcdefABCDEF (base 12) as your character set you can easily convert an integer into that character string, and then back out again. It's useful for taking long numbers and converting them to shorter character strings. I use it convert 10 digit (base 10) numbers into 5 character base 62 strings.

BaseX: Code & Output