$array["720p"] = array("bes","alti","yedi");
$array["subtitle"] = "Ogren";
$array["nosubtitle"] = "Yok";
$array["tr"] = "Yok";


$serialize = serialize(array_merge($array,array("720p"=>true)));
/** veya
$serialize = serialize(array_merge($array,array(key($array)=>true)));
*/

$partserialize = serialize($array["720p"]);
/** veya
$partserialize = serialize($array[key($array)]);
*/