After performing a transaction, Paymob-Accept servers invoke transaction callbacks which in role notify your server with the detail of the performed transaction, check this link to learn more about transaction callbacks.

HMAC is a calculation you perform in order to verify that the post request you just received is from Paymob-Accept's servers end.

Depending on the callback you're expecting to receive, you will perform your calculation, to learn more please check our HMAC calculation found in this Link.

Also, we prepared an HMAC testing endpoint, which response back with the related HMAC concatenated string and the calculated hash related to your transaction ID, check this postman doc, and the attached PDF API doc.