我跑的系統AppServ 2.4.4a php4.3.11

<?php
$GLOBALS["countbytes"]=true;
if (!isset($countbytes))
$GLOBALS["countbytes"] = t2;

echo $GLOBALS["countbytes"];
?>

run to IE：1
---------------------------------
<?php
$GLOBALS["countbytes"]=NULL;
if (!isset($countbytes))
$GLOBALS["countbytes"] = t2;

echo $GLOBALS["countbytes"];
?>

run to IE：t2
----------------------------------

<?php
$GLOBALS["countbytes"]=3;
if (!isset($countbytes))
$GLOBALS["countbytes"] = t2;

echo $GLOBALS["countbytes"];
?>

run to IE：3
-----------------------------------

問題來了，為何跑 false 會什麼都沒有
<?php
$GLOBALS["countbytes"]=false;
if (!isset($countbytes))
$GLOBALS["countbytes"] = t2;

echo $GLOBALS["countbytes"];
?>

run to IE：什麼都沒有
-------------------------------------
後我又把isset前面的 ! 去掉，又正常
<?php
$GLOBALS["countbytes"]=false;
if (isset($countbytes))
$GLOBALS["countbytes"] = t2;

echo $GLOBALS["countbytes"];
?>


run to IE：t2

何解，為何用false跑IE會什麼都沒有.....是isset的bug嗎

please