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<?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) { $result = array(); for($i = 0; $i < $count; $i++) { $pos = $index % strlen($letters); $result[] = $letters[$pos]; $index = ($index-$pos)/strlen($letters); $letters = substr($letters, 0, $pos) . substr($letters, $pos+1); } sort($result); return $result; } $letters = 'abc'; $r = array(); //echo '1 letters from 4: '; for($i = 0; $i < getPermCount($letters, 1); $i++) $r[] = getPerm($letters, 1, $i); //echo '2 letters from 4: '; for($i = 0; $i < getPermCount($letters, 2); $i++) $r[] = getPerm($letters, 2, $i); //echo ' 3 letters from 4: '; for($i = 0; $i < getPermCount($letters, 3); $i++) $r[] = getPerm($letters, 3, $i); print_r(array_unique($r, SORT_REGULAR));