If you have received an email from PayPal with the subject : IMMEDIATE ATTENTION REQUIRED: PayPal service upgrades, don’t panic ??
PayPal is upgrading the certificate for https://www.paypal.com to SHA-256. This endpoint is also used by merchants using the Instant Payment Notification (IPN) product.
Given the types of changes that PayPal has made (or will make) and how our products integrate with it, there’s nothing special that needs to be done on your WordPress site. The Event Registration itself requires no changes.
However, your site may need an updated SSL cert or your server might need updating if it’s not sha-256 compatible. For that, you may need to check this with your host.
The PayPal Sandbox has already been updated to SHA-256. You can test your own site by doing a test registration/payment using the PayPal Sandbox You can quickly test if it will work by making a PayPal Sandbox payment. If you don’t see an error like “No trusted certificate found“ then it should work.
You can also log into the Paypal developer site, Trigger a simulated payment to view payment notifications instantly. Confirm that fields are being parsed correctly and that your handler is working correctly.
https://developer.paypal.com/developer/ipnSimulator/
To quote PayPal: “Any tests that are currently run against PayPal Sandbox endpoints will require a VeriSign G5 root certificate, so you can test your upgrades by making requests against the Sandbox”