Fatal error: Uncaught OAuthException: An active access token must be used

gistfile1.php

<?php
  $app_id = "YOUR_APP_ID";
  $app_secret = "YOUR_APP_SECRET";
  $my_url = "YOUR_POST_LOGIN_URL";
    
  // known valid access token stored in a database
  $access_token = "YOUR_STORED_ACCESS_TOKEN";

  $code = $_REQUEST["code"];
  
  // If we get a code, it means that we have re-authed the user
  //and can get a valid access_token.
  if (isset($code)) {
    $token_url="https://graph.facebook.com/oauth/access_token?client_id=
      . $app_id . "&redirect_uri=" . urlencode($my_url)
      . "&client_secret=" . $app_secret
      . "&code=" . $code . "&display=popup";
    $response = file_get_contents($token_url);
    $params = null;
    parse_str($response, $params);
    $access_token = $params['access_token'];
  }

 
  // Attempt to query the graph:
  $graph_url = "https://graph.facebook.com/me?"
    . "access_token=" . $access_token;
  $response = curl_get_file_contents($graph_url);
  $decoded_response = json_decode($response);

  //Check for errors
  if ($decoded_response->error) {
  // check to see if this is an oAuth error:
    if ($decoded_response->error->type== "OAuthException") {
      // Retrieving a valid access token.
      $dialog_url= "https://www.facebook.com/dialog/oauth?"
        . "client_id=" . $app_id
        . "&redirect_uri=" . urlencode($my_url);
      echo("<script> top.location.href='" . $dialog_url
      . "'</script>");
    }
    else {
      echo "other error has happened";
    }
  }
  else {
  // success
    echo("success" . $decoded_response->name);
    echo($access_token);
  }

  // note this wrapper function exists in order to circumvent PHP’s
  //strict obeying of HTTP error codes.  In this case, Facebook
  //returns error code 400 which PHP obeys and wipes out
  //the response.
  function curl_get_file_contents($URL) {
    $c = curl_init();
    curl_setopt($c, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($c, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($c, CURLOPT_URL, $URL);
    $contents = curl_exec($c);
    $err  = curl_getinfo($c,CURLINFO_HTTP_CODE);
    curl_close($c);
    if ($contents) return $contents;
    else return FALSE;
  }
?>