Code optimizations in PHP

<?php
  // cache-array has the initial value empty for 0 guests
  $variation_cache = array(0 => array());
  // cache will be passed on as pointer not value
  // so we can have the changes in the caller code
  function calc_room_use ($passengers, &$variation_cache) {
    if ($passengers < 1) {
      return 0;
    }
    for ($rn = 1; $rn <= 4; $rn += 1) {
      if ($passengers >= $rn) {
        // we calculate parts only if they were not calc'd already
        if (! isset($variation_cache[$passengers - $rn])) {
          calc_room_use($passengers - $rn, $variation_cache);
        }
        // actual variation is initially empty
        if (! isset($variation_cache[$passengers])) {
          $variation_cache[$passengers] = array();
        }
        $actual_variation = $rn;
        // part-variation-set should be available at this point
        foreach ($variation_cache[$passengers - $rn] as $variation) {
          $actual_variation = explode(',', $variation);
          $actual_variation[] = $rn;
          sort($actual_variation);
          $actual_variation = implode(',', $actual_variation);
          // checks are also done in the cache
          if (! in_array($actual_variation, $variation_cache[$passengers])) {
            // and we also write into
            $variation_cache[$passengers][] = $actual_variation;
          }
        }
        if ($passengers == $rn && ! in_array($rn, $variation_cache[$passengers])) {
          $variation_cache[$passengers][] = $rn;
        }
      }
    }
  }
  calc_room_use($passengers, $variation_cache);
  // variation numbers have to be calculated from the cache,
  // not the function's return value
  echo count($variation_cache[$passengers]);
?>