/**
 * Convert a string to a string in another base
 *
 * Note: based on http://php.net/manual/en/function.base-convert.php#55204
 * Note: the bases are only limited to the length of $sFromMap and $sToMap and $sNumber can be any length
 * 
 * @param string $sNumber  the number which to convert
 * @param int $iFromBase  the base from which to convert
 * @param int $iToBase  the base to which to convert
 * @param string $sFromMap  the ascii string to decode the string
 * @param string $sTomMap  the ascii string to encode the string
 * @return string
 */
function baseConvertString($sNumber, $iFromBase, $iToBase = null, 
	$sFromMap = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_",
	$sToMap   = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_"
) {
	if ($iToBase === null) {
		// use the max
		$iToBase = strlen($sToMap);
	}
	$sNumber = (string) $sNumber;
	$length = strlen($sNumber);
	$result = '';
	for ($i = 0; $i < $length; $i++) {
		$aDigits[$i] = strpos($sFromMap, $sNumber{$i});
	}
	do { // Loop until whole number is converted
		$divide = 0;
		$newlen = 0;
		for ($i = 0; $i < $length; $i++) { // Perform division manually (which is why this works with big numbers)
			$divide = $divide * $iFromBase + $aDigits[$i];
			if ($divide >= $iToBase) {
				$aDigits[$newlen++] = (int)($divide / $iToBase);
				$divide = $divide % $iToBase;
			}	elseif ($newlen > 0) {
				$aDigits[$newlen++] = 0;
			}
		}
		$length = $newlen;
		$result = $sToMap{$divide} . $result; // Divide is basically $sNumber % $iToBase (i.e. the new character)
	} while ($newlen != 0);
	return $result;	
}

base-convert

Convert a string from one base to another

convert

PHP