What are the differences between using htmlspecialchars() and PHP filter methods for input validation, and which flags provide the most restrictive sanitization?

When it comes to input validation and sanitization in PHP, htmlspecialchars() is primarily used to escape special characters in a string to prevent XSS attacks, while PHP filter methods like filter_var() provide more comprehensive filtering options such as validating email addresses, URLs, and integers. The flags FILTER_SANITIZE_STRING and FILTER_SANITIZE_FULL_SPECIAL_CHARS provide the most restrictive sanitization for filter_var(), ensuring that only valid string data is accepted with special characters encoded.

// Using filter_var() with FILTER_SANITIZE_FULL_SPECIAL_CHARS flag for strict input sanitization
$input = '<script>alert("XSS attack")</script>';
$sanitized_input = filter_var($input, FILTER_SANITIZE_FULL_SPECIAL_CHARS);
echo $sanitized_input;