网站黑客的用户名，附加信息和密码更改的问题

我想知道是什么允许一个用户入侵我的网站，他们改变了我的用户名，个人信息和密码。有人能给我一些建议吗?我使用PHP MySQL和HTMLPURIFIER。

这是登录脚本。

<?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 '<p>Your username or email address cannot exceed 255 characters!</p>';
    } else {    
        $e = FALSE;
        echo '<p>You forgot to enter your username or email address!</p>';
    }
    // Validate the password:
    if (!empty($_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 '<p>Your password cannot exceed 255 characters!</p>';
    } else {
        $p = FALSE;
        echo '<p>You forgot to enter your password!</p>';
    }
    if(($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: $pass_salt'n<br />MySQL Error: " . mysqli_error($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 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: $q'n<br />MySQL Error: " . mysqli_error($dbc));

        if (@mysqli_num_rows($r) == 1) { // A match was made.
            // 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/index.php'; // Define the URL:
            header("Location: $url");
            exit(); // Quit the script.
        } else { // No match was made.
            echo '<p>Either your username, email address or password entered do not match those on file or you have not yet activated your account.</p>';
        }
    } else { // If everything wasn't OK.
        echo '<p>Please try again.</p>';
    }
    mysqli_close($dbc);
} // end of submit conditional.
?>