Header Processing

I was looking at an application with the following code snippet:

ob_start();
session_name('foo');
session_start();
if (!$_SESSION['bar']) {
header("Location: http://"; . $_SERVER['HTTP_HOST'] .
dirname($_SERVER['PHP_SELF']) . "/index.php");
ob_end_clean();
exit();
}

I am under the impression that the "header" redirection is immediate.
Am I correct or will the 'ob_end_clean()' and 'exit()' actually
process.

Tom
.