Why isn't my PHP redirect working?

I have my code set to redirect to the main page once the user has logged in successfully, however, it is not working at all.

Could anyone help me out? Here's the code:

<?php 
require("config.php"); 
$submitted_username = ''; 
if(!empty($_POST)){ 
    $query = " 
        SELECT 
            id, 
            username, 
            password, 
            salt, 
            email 
        FROM users 
        WHERE 
            username = :username 
    "; 
    $query_params = array( 
        ':username' => $_POST['username'] 
    ); 

    try{ 
        $stmt = $db->prepare($query); 
        $result = $stmt->execute($query_params); 
    } 

    catch(PDOException $ex){ die("Failed to run query: " . $ex->getMessage()); } 
    $login_ok = false; 
    $row = $stmt->fetch(); 
    if($row){ 
        $check_password = hash('sha256', $_POST['password'] . $row['salt']); 
        for($round = 0; $round < 65536; $round++){
            $check_password = hash('sha256', $check_password . $row['salt']);
        } 
        if($check_password === $row['password']){
            $login_ok = true;
        } 
    } 

    if($login_ok){ 
        unset($row['salt']); 
        unset($row['password']); 
        $_SESSION['user'] = $row;  
        header("Location: index.php"); 
    } 
    else{ 
        print("Login Failed."); 
        $submitted_username = htmlentities($_POST['username'], ENT_QUOTES, 'UTF-8'); 
    } 
} 
 ?> 

The database connection works fine, as in, users are able to register and their details are saved, it's just this block that I'm having problems with.

Thanks!