奇怪的 PHP 语句，求解惑。

$f = "../upload/tmp/20180525/20180525132453802.png";
register_shutdown_function("ffc1");
function ffc1()
{
global $f;
if(is_file($f))echo "yyy\n";
else echo "nnnn\n"; $ff = "F:/tmp/wifi.txt";
if(is_file($ff))echo "yyy\n";
else echo "nnnn\n"; if(is_file($f))echo "yyy\n";
else echo "nnnn\n";

}
if(is_file($f))
{

}
/*
奇怪的输出:
yyy
yyy
nnnn
*/