有缺陷。
改前输出：This is a string,
改后输出：This is a string, and something extra.
function add_some_extra($string)
{
$string .= 'and something extra.';
return $string;
}
$str = 'This is a string, ';
$str = add_some_extra($str);
echo $str;
输出'This is a string, and something extra.'
