I need to show an alert box to the user in case a form gets successfully submitted.

But it is not working. The alert box does'nt pop up.

The form is on page contactus.php. I want the user to stay on that page only. So i have added

header('Location: contactus.php');

Here is the code.

PHP Code:
$qry "insert into contactus(name,email,subject,message) values('$name','$email','$subject','$message')";
$result=mysql_query($qry);
if(
$result)
{

echo 
"<script type='text/javascript'>\n";
echo 
"alert('Your message was successfully sent.');\n";
echo 
"</script>";
header('Location: contactus.php');
}
else
{
echo 
"<script type='text/javascript'>\n";
echo 
"alert('Could'nt send your message.');\n";
echo 
"</script>";
header('Location: contactus.php');