$app_id='MY APP ID';
$app_secret='MY APP SECRET';
$access_token="ACCESS TOKEN";
$url="https://graph.facebook.com/oauth/access_token?client_id=".$app_id."&client_secret=".$app_secret."&grant_type=fb_exchange_token&fb_exchange_token=".$access_token;
 $a=file_get_contents($url);
 echo $a;

{
   "error": {
      "message": "Error validating client secret.",
      "type": "OAuthException",
      "code": 1
   }
}