PHP empty()函數：Can't use method return value in write context

<?php
if (!empty (get_gpc('userId')))
{
	$userId = get_gpc('userId');
}
else
{
	$error = "ID doesn't exist";
}

報錯：

Fatal error: Can't use method return value in write context in (line number)

為什么？

empty()函數是檢查一個變量是否為空，但是 get_gpc() 是個函數，所以得改下，參考代碼如下：

<?php
$test = get_gpc('userId');
if (!empty($test))
{
	$userId = get_gpc('userId');
}
else
{
	$error = "ID doesn't exist";
}