aspetta però per l'id devo sempre utilizzare una $_GET? esempio:

<?php

include 'connessione.php';

$id = $_GET['id']; <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

$query = "SELECT id FROM table WHERE id = '$id'";

$result = mysql_result($query, $db_conn);
$num_rows = mysql_num_rows($result);

?>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento senza titolo</title>
</head>

<body>


Ecco il mio dato: <? echo $id ?>
</body>
</html>