PHP empty()函数：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";
}

报错：

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

为什么？

empty()函数是检查一个变量是否为空，但是 get_gpc() 是个函数，所以得改下，参考代码如下：

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