What are the potential security risks associated with using nl2br() and htmlentities in PHP forms?

Using nl2br() and htmlentities in PHP forms can help prevent XSS attacks by converting special characters to their HTML entities and preserving line breaks. However, it's important to be cautious when using these functions as they may not provide complete protection against all types of attacks. It's recommended to also validate and sanitize user input before using these functions to further enhance security.

// Validate and sanitize user input before using nl2br() and htmlentities
$user_input = $_POST['user_input'];

// Validate and sanitize user input
$validated_input = filter_var($user_input, FILTER_SANITIZE_STRING);

// Apply nl2br() and htmlentities to the sanitized input
$safe_input = nl2br(htmlentities($validated_input));