Refresh Question

[182x]

Hey guys, I have created a page which had the PHP as listed below and the page works fine however if the user clicks refresh the pages goes all white and blank. I was just wondering why it does this? Thanks

<?php
		session_start();
		include ('db.php');
		$link_id = db_connect('');

$g = "SELECT ff, nn e FROM cu WHERE Id = '". $HTTP_SESSION_VARS ["Id"]."'";

$rs = mysql_query($g, $link_id);


if (mysql_num_rows($rs) > 0 )

	{
		$r = mysql_fetch_assoc($rs);
		$HTTP_SESSION_VARS ["Id"] = $r["Id"];

	}


else
	{
		echo $HTTP_SESSION_VARS ["Id"];
		exit();
	}

?>