When passing variables through the URL in PHP, what are some best practices for handling and processing this data?

When passing variables through the URL in PHP, it is important to properly sanitize and validate the data to prevent security vulnerabilities such as SQL injection or cross-site scripting attacks. One best practice is to use PHP's built-in functions like htmlspecialchars() to escape special characters and prevent code injection. Additionally, you should always validate and filter the input data to ensure it meets the expected format before processing it further. 

// Example of sanitizing and validating URL parameters in PHP
$userId = isset($_GET['user_id']) ? intval($_GET['user_id']) : null;

if ($userId) {
    // Process the user ID
} else {
    // Handle invalid or missing user ID
}

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