Something

 $sid=false;
  $u=unpack("H2rev/H2b/nc/Nd/V*e", $osid);
  if ($u) {
    $n232=pow(2,32);
    unset($u["b"]); // unused
    $u["c"]= $n232*$u["c"]+$u["d"];
    unset($u["d"]);
    $sid="S";
    foreach ($u as $v) {
      if ($v < 0) $v=$n232 + $v;
      $sid.= "-".$v;
    }
  }
  return $sid;
}