Something

<?php 
$letters = range('a', 'z');
permutate($letters, $results, 3);
echo var_dump($results);

function permutate(&$letters, &$results = array(), $length, $string = "")
{
	if ($length == 0) {
		$cnt = count($results);
		$results[] = $string;
		return;
	}
	
	foreach ($letters as $l) {
		permutate($letters, $length - 1, $string . $l);
	}
}
?>