PHP empty() 出現 Fatal error: Can't use function return value in write context 錯誤訊息

因為 PHP 5.4 以前，empty() 只能處理變數。
若直接在  empty() 放函式，會出現 Fatal error: Can't use function return value in write context 錯誤

function aa(){
   return 123;
}

empty(aa()); // Fatal error: Can't use function return value in write context 

$a = aa();
empty($a); // false 

官網說明：
http://php.net/manual/en/function.empty.php
Prior to PHP 5.5, empty() only supports variables; anything else will result in a parse error. In other words, the following will not work: empty(trim($name)). Instead, use trim($name) == false.