Logged In user switching to another user in PHP session
<?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 = 'Page Title';
include ('../includes/header.php');

$mysqli = mysqli_connect("localhost", "some", "some", "some");

if(isset($_SESSION['user_id'])) {

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

//HTML Purifier
require '../htmlpurifier/library/HTMLPurifier.auto.php';
//End HTML Purifier

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 'Error';
    } else {
        $e = FALSE;
        echo 'Error';
    }

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

    if(($e != FALSE) && ($p != FALSE)) { // check pass
        $pass_salt = "SELECT users.password, users.salt FROM users JOIN contact_info ON contact_info.user_id = users.user_id WHERE (contact_info.email = '" . $e . "' OR users.username = '" . $e . "') AND users.active IS NULL";
        $ph = mysqli_query($dbc, $pass_salt) or trigger_error("Query: $pass_saltn<br />MySQL Error: " . mysqli_error($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.
        $q = "SELECT users.user_id, users.first_name, users.user_level FROM users JOIN contact_info ON contact_info.user_id = users.user_id WHERE (contact_info.email = '" . $e . "' OR users.username = '" . $e . "') AND users.password = '" . $sha512 . "' AND users.active IS NULL";
        $r = mysqli_query ($dbc, $q) or trigger_error("Query: $qn<br />MySQL Error: " . mysqli_error($dbc));

        if (@mysqli_num_rows($r) == 1) {

            // Register the values & redirect:
            $_SESSION = mysqli_fetch_array ($r, MYSQLI_ASSOC);
            // check if user is logged in then update the old login date
            $u = "UPDATE users JOIN contact_info ON contact_info.user_id = users.user_id SET users.last_login = NOW(), users.deletion = 0, users.deletion_date = NULL WHERE (contact_info.email = '" . $e . "' OR users.username = '" . $e . "') AND users.password = '" . $sha512 . "' AND users.active IS NULL";
            // save the info to the database
            $r = mysqli_query ($dbc, $u);
            mysqli_free_result($r);
            mysqli_close($dbc);

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

        } else { // No match was made.
            echo 'Error';
        }

    } else { // If everything wasn't OK.
        echo 'Error';
    }
    mysqli_close($dbc);
}
?>

<?php
ob_start(); // Start output buffering. // This is the logout page for the site.
session_start(); // Initialize a session.

require_once ('../includes/config.inc.php');
$page_title = 'Title';

// If no user_id session variable exists, 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.

} else { // Log out the user.

    $_SESSION = array(); // Destroy the variables.
    session_destroy(); // Destroy the session itself.
    setcookie(session_name(), '', time() - 2592000, '/'); // Destroy the cookie.

}

$url = BASE_URL;
ob_end_clean();
header("Refresh: 3; $url");
include ('../includes/header.php');

$mysqli = mysqli_connect("localhost", "some", "some", "some");

include ('../includes/footer.php');
exit(); // Quit the script.
?>

ob_start();// Start output buffering.
session_start();// Initialize a session.

// Set the page title and include the HTML header.
$page_title = 'Title';
include ('../includes/header.php');

// Include the configuration file for error management and such.
require_once ('../includes/config.inc.php');
require_once ('../mysqli_connect.php'); // Connect to the db.

$mysqli = mysqli_connect("localhost", "some", "some", "some");

// If no user_id session variable exists, 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.
}

...WHERE
    (contact_info.email = '" . $e . "' OR users.username = '" . $e . "')
AND
    users.password = '" . $sha512 . "'
AND
    users.active IS NULL"