Getting Facebook username using Graph API v2.0

facebook.php

<?php
  /**
   * file_get_contents
   */
  // Get User ID from FB SDK, this example uses Facebook PHP SDK v4
  $fb_userID = $fb_data['me']['id'];
  $fb_graph = 'https://graph.facebook.com/'.$fb_userID;
  $fb_request = file_get_contents($fb_graph); // Replace with curl_get_contents if this does not work
  $fb_response = json_decode($fb_request);
  // Print username
  print $fb_response->username;
  /**
   * CURL (Mediatemple)
   */
  public function curl_get_contents( $url ) {
    $request = curl_init();
    
    curl_setopt($request, CURLOPT_AUTOREFERER, TRUE);
    curl_setopt($request, CURLOPT_HEADER, 0);
    curl_setopt($request, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($request, CURLOPT_URL, $url);
    curl_setopt($request, CURLOPT_FOLLOWLOCATION, TRUE);       
    $data = curl_exec($request);
    
    curl_close($request);

    return $data;
  }
?>

Above code will not work until pass access token like below:
$fb_graph = 'https://graph.facebook.com/'.$fb_userID."?access_token=YOUR_ACCESS_TOKEN";

not working