base_convert($your_string, $from_base, $to_base) - This PHP function converts a number between two bases. Supported bases includes base-2 through base-36. Digits in numbers with a base higher than 10 will be represented with the letters (a through z), with the letter a to mean 10, the letter b to mean 11 and the letter z to mean 35.
Note that base_convert() function may lose precision on large numbers due to the properties related to the internal "double" or "float" type used.