Something

<?php$set = array(1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12);$n = count($set);$k = 2;function subsets($i, $j, $subset, $lambda) {   global $set;   global $n;   global $k;   if ($j == $k) {      if ($lambda($subset[0], $subset[1]) == 5) {         print_array($subset);      }      return;   }   while ($i < $n) {      array_push($subset, $set[$i]);      subsets($i, $j + 1, $subset, $lambda);      array_pop($subset);      ++$i;   }}function print_array($array) {   $end = count($array) - 1;   echo "{";   for ($i = 0; $i <= $end; $i++) {      if ($i != $end) {         echo "$array[$i], ";      } else {         echo "$array[$i]}<br>\r\n";      }   }}$serie = create_function('$a,$b', 'return $a + $b;');$parallel = create_function('$a,$b', 'return 1/((1/$a) + (1/$b));');subsets(0, 0, array(), $serie);?>