facebook extended access token

$token_common = $facebook->getAccessToken();
$fb_appid = "xxx";
$fb_appsecret = "xxx";
$urltoken = "https://graph.facebook.com/oauth/access_token?client_id=$fb_appid&client_secret=$fb_appsecret&grant_type=fb_exchange_token&fb_exchange_token=$token_common";
$graph = file_get_contents($urltoken);
$graph = explode("=", $graph);
$graph = explode("&", $graph[1]);
$extended_alt_token = $graph[0];