PHP check for cookies doesn't work

Hi there,
here is my problem:

I successfully wrote a cookie. Then I check for this cookie with Java Script and PHP code. But only Java Script returns me “found”.

<script language="javascript">
	<!--
	if (document.cookie.indexOf("users_res") >= 0) {
  alert("found");
}
else {
  alert("not found");
}
	//-->
	</script>
<?php
if (isset($HTTP_COOKIE_VARS["users_res"]))  {
    echo "found";
} else {
    echo "not found";
}
?>

When I put this PHP code into test.php and I upload it manually to my site – PHP script shows “found”. But when I try to insert the code into header.php or any other PHP file of my WP theme – it shows “not found”.

I tested this PHP code with 2 other sites with different themes and WP versions, but no change.