how to solve IPN fatal error for paypal transaction?

Question

I'm having an eCommerce system "Zen Cart" Installed with website PayPal IPN for payment module, and i've set it to sandbox to test the functionality of the purchase.

after i checkout to PayPal and i pay with a sandbox account to a sandbox business test account and the money actually goes to business account, it returns to the website with no number of order displayed, and no orders in the admin panel or anywhere.

the log showed this:

Apr 09 2013 11:53 -- IPN INFO - Confirmation/Validation response 



Apr 09 2013 11:53 -- Breakpoint: 1 - Collected data from PayPal notification

Apr 09 2013 11:53 -- IPN WARNING :: Transaction was not marked as VERIFIED. Keep this report for potential use in fraud investigations.
IPN Info = 


Apr 09 2013 11:53 -- IPN FATAL ERROR :: Transaction did not validate

When i do ipncheck.php of the zen cart i get this error:

IPNCHECK.PHP - Version 1.0

IPN TESTING - POSTING to PayPal via: ssl://www.paypal.com:443

IPN TESTING - Confirmation/Validation response: RESPONSE RECEIVED - Communications OKAY


Script finished.

Please need help with this!

Thanks in advance!

UPDATE:

i'm getting this error in the main errors log: [10-Apr-2013 13:54:08] PHP Warning: implode() [<a href='function.implode'>function.implode</a>]: Invalid arguments passed in includes/modules/payment/paypal/paypal_functions.php on line 506

Answer 1

It sounds like you are receiving the communication, but the POST when being sent back to PayPal for validation is not validating. Causing the order not to get updated. Take a look at the IPN troubleshooting steps here, they may help. Log the request that you are receiving, and the one that your system is sending back and compare them. See if there is any difference.