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For the easiest work download the integration hook and unzip it into the /includes/hooks/ directory.
In the uploaded pap.php file follow the instructions on lines 10-12.
Do not forget to upload the latest PapApi.class.php file to the /includes/hooks/ directory as well. The PapApi.class.php file can be downloaded from the merchant panel of Post Affiliate Pro (Network) at Tools> Integration> API Integration> Download PAP API.