How should a complex, multi step form, such as a checkout process, handle validation and state management across multiple pages while maintaining data integrity and security?

A robust multi step form typically validates each individual step's data as the user progresses, storing already validated data temporarily in the session so it persists across steps without needing to resubmit everything at once, performs a final comprehensive validation of all collected data before actually processing the complete submission at the end, and carefully guards against a user attempting to skip directly to a later step without properly completing the required earlier steps first.

// Step 1: validate and store in session
$_SESSION['checkout']['shipping'] = validateShippingData($_POST);
// Final step: validate entire session data before processing order
$order = validateAndCreateOrder($_SESSION['checkout']);