WHY IS THIS PHP SCRIPT NOT WORKING?

//test.php

if (isset($_POST['submit'])) {
if ($_POST['username']) == 'myusername') && ($_POST['password']) == 'mypassword')) {
header ('location: search.php');
exit();
}
}
?>

i wish to redirect the user to my search engine if their login was successful.

thanks a bunch.
whilikers, that is a great point, thanks. I have made the capitalization, but unfortunately it still does not work. something somewhere is wrong:-)
ADD:

what does it mean “if your web page receives any html or even blank space, the header() function wont work”?