What improvements can be made to the existing IPN script to enhance its security and efficiency, particularly in relation to handling POST variables and data validation?

Issue: The existing IPN script may be vulnerable to security risks and inefficiencies due to inadequate handling of POST variables and lack of proper data validation. To enhance security and efficiency, it is crucial to sanitize and validate incoming POST data to prevent SQL injection, cross-site scripting, and other malicious attacks. 

// Improved IPN script with enhanced security and efficiency

// Sanitize and validate incoming POST data
foreach ($_POST as $key => $value) {
    $_POST[$key] = htmlspecialchars(strip_tags(trim($value)));
}

// Validate required fields
if (!isset($_POST['payment_status']) || !isset($_POST['txn_id'])) {
    // Handle missing required fields
    exit('Error: Missing required fields');
}

// Process the IPN data
// Your existing IPN processing code goes here

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