Question

In paypal’s IPN sample code for PHP5.2, on the last lines, they got a comment to “log for manual investigation”.

else if (strcmp ($res, "INVALID") == 0) {
    // log for manual investigation
}

What kind of info should I log and how ?

Answer 1

Well you have to log the answer you got from paypal via the $_POST vars.

If you use a framework or CMS it might have a logging function. Otherwise you can use PHP’s error_log function

For example:

$post_vars = var_export($_POST, true);
error_log("IPN Result: " . $res, 3, "/var/tmp/ipn.log");
error_log("POST_VARS: " . $post_vars, 3, "/var/tmp/ipn.log");

Where /var/tmp/ipn.log is a custom path where your log file should appear.