How does the payment form redirect to thank you page?

I have set top a php to be called from thank you and also a php from paypal.

If my users go to paypal then the thank you script does not get called.

When your form has a payment field, the thank you page will be processed after the successful payment.

Example, the user will be directed to Paypal checkout page (or donation page), after the successful payment, Paypal would redirect back to your form and proceed with the thank you page. If it's a custom thank you URL, it would redirect to that URL.