Alphabet permutations without arrays or recursion

gistfile1.php

<?php

$string_len = 5;

$alphabet = 'abc';
$alphabet_len = strlen($alphabet);

for ($s = 1; $s <= $string_len; $s++) {
	$current_key = str_repeat($alphabet[0], $s);

	$num_combinations = pow($alphabet_len, $s);
	$i = 0;
	while ($i++ < $num_combinations) {
		printf('%s<br />', $current_key);
		
		$alphabet_index = $i % $alphabet_len;
		$current_key[0] = $alphabet[$alphabet_index];
		
		if (($alphabet_index === 0) && ($s !== 1)) {
			$key_index = 1;
			$alphabet_index = (strpos($alphabet, $current_key[$key_index]) + 1) % $alphabet_len;
			$current_key[$key_index] = $alphabet[$alphabet_index];
			printf('$current_key[%d] = $alphabet[%d], %d = %s<br />', $key_index, $alphabet_index, $key_index, $alphabet[$alphabet_index]);
			
			if ($alphabet_index === 0) {
				$key_index = ($key_index + 1) % $s;
				while ($key_index > 0) {
					$alphabet_index = (strpos($alphabet, $current_key[$key_index]) + 1) % $alphabet_len;
					$current_key[$key_index] = $alphabet[$alphabet_index];
					printf('$current_key[%d] = $alphabet[%d], %d = %s<br />', $key_index, $alphabet_index, $key_index, $alphabet[$alphabet_index]);
					$key_index = ($key_index + 1) % $s;
					
					if ($alphabet_index !== 0) break;
				}
			}
		}
	}

}