/**************
This code is public domain.

This function increments a case-sensitive alphanumeric
number.  It will use as many digits as you give it.  If
it tries to increment a number that is at it's maximum
(eg "ZZ"), it returns boolean false.
**************/

define('NUMBER_START', ord('0'));
define('NUMBER_END', ord('9'));
define('LOWER_START', ord('a'));
define('LOWER_END', ord('z'));
define('UPPER_START', ord('A'));
define('UPPER_END', ord('Z'));

function incrementNumber($number) {
	if (strlen($number) == 0)
		return false;
	
	$number = str_split($number);
	$last = ord(array_pop($number));
	switch ($last) {
		case NUMBER_END:
			$last = LOWER_START;
			break;
		case LOWER_END:
			$last = UPPER_START;
			break;
		case UPPER_END:
			$last = NUMBER_START;
			$inc = incrementNumber(implode($number));
			if ($inc === false)
				return false;
			$number = str_split($inc);
			break;
		default:
			$last++;
	}
	$last = chr($last);
	$number[] = $last;
	return implode($number);
}

// Usage
echo incrementNumber('0000'); // outputs "0001"
echo incrementNumber('0aB4'); // outputs "0aB5"
echo incrementNumber('0aB9'); // outputs "0aBa"
echo incrementNumber('0aBz'); // outputs "0aBA"
echo incrementNumber('0aBZ'); // outputs "0aC0"