What are some common pitfalls to avoid when passing variables through URLs in PHP?

One common pitfall to avoid when passing variables through URLs in PHP is not properly sanitizing user input, which can lead to security vulnerabilities such as SQL injection or cross-site scripting attacks. To mitigate this risk, always validate and sanitize any input coming from the URL before using it in your code.

// Example of sanitizing input from a URL variable
$user_id = isset($_GET['user_id']) ? filter_var($_GET['user_id'], FILTER_SANITIZE_NUMBER_INT) : null;