Gracias :D Lo he resuelto con
Código:
$result_email = mysql_query("SELECT email FROM $db5.users WHERE email='$email'");
$result_username = mysql_query("SELECT * FROM $db5.users WHERE username='$username'");
$result_email = mysql_num_rows($result_email);
$result_username = mysql_num_rows($result_username);
if($result_email == 0 && $result_username == 0)
{
codigo
}