解法：
session_start();
if($_GET["state"){
$_SESSION["FBRLH_state"]=$_GET['state'];
}
參考：
https://github.com/facebook/facebook-php-sdk-v4/issues/473
https://github.com/facebook/facebook-php-sdk-v4/blob/4.0.16/src/Facebook/FacebookRedirectLoginHelper.php#L205
https://github.com/facebook/facebook-php-sdk-v4/issues/470#issuecomment-212561488
https://github.com/facebook/facebook-php-sdk-v4/issues/572
https://github.com/facebook/facebook-php-sdk-v4/issues/470
https://github.com/facebook/facebook-php-sdk-v4/issues/529
