Un include segun opción elegida

[Zidencjb]

$query = mysql_query("SELECT username,password FROM users WHERE username = '$username'") or die(mysql_error());
$row = mysql_fetch_array($query);
$_SESSION["s_username"] = $row['username];

Puede ser por

$query = mysql_query("SELECT username,password,opcion FROM users WHERE username = '$username'") or die(mysql_error());
$row = mysql_fetch_array($query);
$_SESSION["s_username"] = $row['username];
if($row[opcion]=='si')
{
header('location: pagina1.php');
}
else if($row[opcion]=='no')
{
header('location: pagina2.php');
}