Facebook访问令牌

我正在尝试使用下面的代码获取Facebook访问令牌：我之前确实创建了自己的函数，但它返回了相同的错误，我无法弄清楚问题是什么。

function getAccessToken(){
        $app_id = FB_APP_ID;
        $app_secret = FB_SECRET_ID;
        $my_url = FB_PAGE_URL;
        $code = $_REQUEST["code"];   

        if(empty($code)) {
       $_SESSION['state'] = md5(uniqid(rand(), TRUE)); //CSRF protection
       $dialog_url = "https://www.facebook.com/dialog/oauth?client_id=" 
         . $app_id . "&redirect_uri=" . urlencode($my_url) . "&state="
         . $_SESSION['state'];
       echo("<script> top.location.href='" . $dialog_url . "'</script>");
     }      
    if($_REQUEST['state'] == $_SESSION['state']) {
       $token_url = "https://graph.facebook.com/oauth/access_token?"
         . "client_id=" . $app_id . "&redirect_uri=" . urlencode($my_url)
         . "&client_secret=" . $app_secret . "&code=" . $code;
       $response = file_get_contents($token_url);
       $params = null;
       parse_str($response, $params);
       $graph_url = "https://graph.facebook.com/me?access_token=" 
         . $params['access_token'];
       $user = json_decode(file_get_contents($graph_url));
       echo("Hello " . $user->name);
     }
     else {
       echo("The state does not match. You may be a victim of CSRF.");
     }
}

取自服务器端身份验证，但我不断收到此错误：

    Warning: file_get_contents(https://graph.facebook.com/oauth/access_token?
client_id=ID HERE&redirect_uri=URLHERE&cli
ent_secret=SECRECT&code=AQCI5rNgw9zCPHWGozeT59asg7_022u5tVc5XSef49BiX
IaF5_MAMqFwsqOAquUHgjOu_99ONwUV6IC7k-jV6DsWf9ni3jm8t59aHCBp1jrFaDthPbIKLNLQ-
fZgB5MLh1le5BAPKj_l57jhTLTBfOdxRU30mFCMYzMch8MYFpCmJ9GrjSSGwt0OKb_LNqMoRf8) [function.file-
get-contents]: failed to open stream: HTTP request failed! HTTP/1.0 400 Bad Request

有人知道解决方法吗？