What are the best practices for accessing session data in PHP to ensure security and maintainability of code?

When accessing session data in PHP, it is important to sanitize and validate the data to prevent security vulnerabilities such as session hijacking or injection attacks. To ensure maintainability of code, it is recommended to create helper functions or classes to handle session data retrieval and manipulation. 

// Sanitize and validate session data before using it
$user_id = isset($_SESSION['user_id']) ? filter_var($_SESSION['user_id'], FILTER_VALIDATE_INT) : null;

// Create a helper function to retrieve session data
function getSessionData($key) {
    return isset($_SESSION[$key]) ? $_SESSION[$key] : null;
}

// Example usage of the getSessionData function
$user_id = getSessionData('user_id');

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