I can't remember much of the P/NP stuff from my comp sci university days.

Wouldn't this work? O(n * k)?

  <?php

  $input = array(-2, -3, 15, 14, 7, -10);

  $sums = array();

  foreach ($input as $val) {

    $i = 0;
    $count = count($sums);
    foreach ($sums as $sum => $bool) {
      if ($i >= ($count / 2)) {
        break;
      }

      $sums[$sum + $val] = true;
    }
	
    $sums[$val] = true;
    if (array_key_exists(0, $sums)) {
      print "yes\n";
      break;
    }
  }