How can implementing IP checks, user registration systems, and email sending limits enhance the security and efficiency of PHP scripts handling form submissions?

Implementing IP checks can help verify the origin of form submissions, user registration systems can ensure that only authenticated users can submit forms, and email sending limits can prevent spam or abuse of the form submission system. These measures can enhance the security and efficiency of PHP scripts handling form submissions by reducing the risk of unauthorized access, improving data integrity, and preventing server overload.

// IP check
$allowed_ips = array('192.168.1.1', '10.0.0.1');
if (!in_array($_SERVER['REMOTE_ADDR'], $allowed_ips)) {
    die('Access denied.');
}

// User registration system
session_start();
if (!isset($_SESSION['user_id'])) {
    header('Location: login.php');
    exit();
}

// Email sending limit
$limit = 100; // Set the limit to 100 emails per hour
$emails_sent = // Retrieve the number of emails sent in the last hour
if ($emails_sent >= $limit) {
    die('Email sending limit exceeded.');
}