How can PHP developers improve the security and readability of their code when working with global variables and handling user input, such as GET parameters in navigation links?

When working with global variables and handling user input like GET parameters, PHP developers can improve security by sanitizing and validating the input to prevent potential security vulnerabilities like SQL injection or cross-site scripting attacks. They can also enhance code readability by using meaningful variable names and comments to explain the purpose of the code.

// Sanitize and validate GET parameter before using it
$user_id = isset($_GET['user_id']) ? filter_var($_GET['user_id'], FILTER_SANITIZE_NUMBER_INT) : null;

// Use meaningful variable names and comments to improve code readability
// Get user ID from GET parameter
$user_id = isset($_GET['user_id']) ? filter_var($_GET['user_id'], FILTER_SANITIZE_NUMBER_INT) : null;