Online PHP editor | output for YtrrC

<?php // Returns the total number of $count-length strings generatable from $letters. function getPermCount($letters, $count) { $result = 1; // k characters from a set of n has n!/(n-k)! possible combinations for($i = strlen($letters) - $count + 1; $i <= strlen($letters); $i++) { $result *= $i; } return $result; } // Decodes $index to a $count-length string from $letters, no repeat chars. function getPerm($letters, $count, $index) { $result = ''; for($i = 0; $i < $count; $i++) { $pos = $index % strlen($letters); $result .= $letters[$pos]; $index = ($index-$pos)/strlen($letters); $letters = substr($letters, 0, $pos) . substr($letters, $pos+1); } return $result; } $letters = 'abcd'; echo '2 letters from 4:<br>'; for($i = 0; $i < getPermCount($letters, 2); $i++) echo getPerm($letters, 2, $i).' '; echo '<br>3 letters from 4:<br>'; for($i = 0; $i < getPermCount($letters, 3); $i++) echo getPerm($letters, 3, $i). " ";