<?php
ob_start();
include_once "functions.php";
include_once "db_config.php";

$name = htmlspecialchars($_POST['name']);
$city = htmlspecialchars($_POST['city']);
$email = htmlspecialchars($_POST['email']);
$url = htmlspecialchars($_POST['url']);
$msg = htmlspecialchars($_POST['msg']);

$time =mysql_query("SELECT NOW();");
$puttime = mysql_result($time,0);
if(!$puttime)
{
$puttime= "0000 00 00 00:00:00";
}
$add_query = "INSERT INTO guest VALUES(0,'$name','$city','$email','$url','$msg','$ puttime','show')";
if (!mysql_query($add_query))
{
echo "cant write message";
ob_flush();
die();
}

header ("Location: http://google.ru");
ob_flush();
die();

?>

попробуй
а вообще до header не должно быть никакого вывода