$a = Array(
  '0' => Array('id' => 3,'sid' => 0),
  '1' => Array('id' => 4,'sid' => 0),
  '2' => Array('id' => 5,'sid' => 3),
  '3' => Array('id' => 6,'sid' => 0),
  '4' => Array('id' => 7,'sid' => 4),
  '5' => Array('id' => 8,'sid' => 0),
);

function cmsort($ar, $sid=0, &$res=array()) {
  foreach($ar as $r) {
    if($r['sid'] == $sid) {
      $res[] = $r;
      cmsort($ar, $r['id'], $res);
    }
  }
  return $res;
}

var_export(cmsort($a));