What are some best practices for handling user-submitted backlink information in PHP to prevent abuse or manipulation of the system?

Issue: When handling user-submitted backlink information in PHP, it is important to implement proper validation and sanitization techniques to prevent abuse or manipulation of the system. This includes validating the URL format, checking for malicious code, and implementing measures to prevent duplicate submissions. PHP Code Snippet:

// Validate the submitted URL format
if(filter_var($user_input_url, FILTER_VALIDATE_URL) === false) {
    // Handle invalid URL error
}

// Sanitize the URL to prevent XSS attacks
$sanitized_url = filter_var($user_input_url, FILTER_SANITIZE_URL);

// Check for duplicate submissions
if($existing_backlinks_query->num_rows > 0) {
    // Handle duplicate submission error
} else {
    // Process the backlink submission
}