PHP - An error occurred on line 100: Cannot modify header information - headers already sent

Question

Before I begin I have 2 headers in my code, it is the last header in my code thats giving me some problems though and I want to know how can I fix my code without having to make major changes to it if possible so that I dont get this error any more?

If it helps I have <? ob_start(); ?> already in my header include and <? ob_flush(); ?> in my footer include.

Here is my php code.

<?php 
// This is the login page for the site.
require_once ('../includes/config.inc.php'); 
// Set the page title and include the HTML header.
$page_title = 'Login - Site Name';
include ('../includes/header.php');

$mysqli = mysqli_connect("localhost", "root", "", "sitename");

//If members is logged in, redirect the user:
if(isset($_SESSION['user_id'])) {

    $url = BASE_URL . 'index.php'; // Define the URL.
    ob_end_clean(); // Delete the buffer.
    header("Location: $url");
    exit(); // Quit the script. 
}

?>

<!-- main content -->
<div id="content">

    <!-- center content -->
    <div id="center-content">

        <!-- left content -->
        <div id="main-content">

            <!-- sign in form -->
            <div class="login-form">
                <h2>Sign In To Your Account</h2>
                <form method="post" action="index.php">
                    <fieldset>

<?php
if (isset($_POST['submitted'])) { // start of submit conditional.
    require_once (MYSQL);

    // Validate the username or email address:
    if (!empty($_POST['login']) && strlen($_POST['login']) <= 255) {
        $e = mysqli_real_escape_string($dbc, $purifier->purify(strip_tags($_POST['login'])));
    } else if(!empty($_POST['login']) && strlen($_POST['login']) >= 256) {
        $e = FALSE;
        echo 'do something else';
    } else {    
        $e = FALSE;
        echo 'do something else';
    }

    // Validate the password:
    if (!empty($_POST['pass']) && strlen($_POST['pass']) <= 255) {
        $p = mysqli_real_escape_string($dbc, $purifier->purify(strip_tags($_POST['pass'])));
    } else if(!empty($_POST['pass']) && strlen($_POST['pass']) >= 256) {
        $p = FALSE;
        echo 'do something else';
    } else {
        $p = FALSE;
        echo 'do something else';
    }

    if(($e != FALSE) && ($p != FALSE)) { // check pass
        $pass_salt = "SELECT from database";
        $ph = mysqli_query($dbc, $pass_salt) or trigger_error("Query: $pass_salt\n<br />MySQL Error: " . mysqli_error($dbc));

        while($row = mysqli_fetch_array($ph)){ 
            $password = $row['password'];
            $salt = $row['salt'];
        }

        if(!empty($salt)) {
            $sha512 = hash('sha512', $p . $salt);
        }

        if(!empty($password) == !empty($sha512)){
            $user_pass = TRUE;
        } else {
            $user_pass = FALSE;
        }
    }


    if(isset($user_pass) && ($user_pass == TRUE) && !empty($salt)) { // If everything's OK.
        // Query the database:
        $q = "SELECT from database";        
        $r = mysqli_query ($dbc, $q) or trigger_error("Query: $q\n<br />MySQL Error: " . mysqli_error($dbc));


        if (@mysqli_num_rows($r) == 1) { // A match was made.

            $_SESSION = mysqli_fetch_array ($r, MYSQLI_ASSOC); 
            $u = "UPDATE database"; 
            // save the info to the database
            $r = mysqli_query ($dbc, $u);
            mysqli_free_result($r);
            mysqli_close($dbc);

            $url = BASE_URL . 'home/index.php'; // Define the URL:
            ob_end_clean(); // Delete the buffer.
            header("Location: $url");
            exit(); // Quit the script.

        } else { 
            echo 'do something else';
        }

    } else { 
        echo 'do something else';
    }

    mysqli_close($dbc);

} // end of submit conditional.
?>