Facebook Graph API. How to get comment from post

Hi i have this to get comment from post it works is there is a years

 $json = file_get_contents("http://ift.tt/1RHTDtg");
    echo $json;

but now i have this error in facebook

{
   "error": {
      "message": "An access token is required to request this resource.",
      "type": "OAuthException",
      "code": 104
   }
}

how i can get all comment from post ?

i have using your code @luschn my request with true APP ID and App Secret

http://ift.tt/1eRiTBa]

i have this

{
   "error": {
      "message": "Invalid OAuth access token signature.",
      "type": "OAuthException",
      "code": 190
   }
}

i use chrome to look this error

-- English is not my native language, sorry for any mistakes.