Help Please Form Post Variable

I have problem, about
post variable of form
why when i submit
and

<?
echo $myvar;
?>

"echo $myvar" doesn't seem to work.

but this "echo $HTTP_POST_VARS["myvar"]"
is worked.

Why
I use apache 1.3.22 on XP 2600
php 4.06 (CGI binary)

How can i solve this case?

My old php on win98 is ok.
i don't wanna use $HTTP_POST_VARS["myvar"] for everytime i catch data from form

thanks

Can you give a little more info?

If you post a slightly more comprehensive script snippet, it would be a little more helpful.

Rss

this.php is the file
when i put value in textfield;
i can't print $xxx

<head>
<title>Untitled</title>
</head>

<body>
<?

echo $xxx

?>
<form action="this.php" method="post">
<input type="text" name="xxx" value="">
<input type="submit">

</form>

</body>