Paypal SandBox IPN always returns INVALID

Question

As mentioned in one of the comments in an answer below, I tried following this tutorial. So now I have the following:

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The ipn.php file:

Answer 1

Here's how to avoid these errors...

foreach ($_POST as $key => $value) {
     if ($key=='transaction')
          foreach ($value as $key2=>$value2) {
               $value['transaction'][$key2] = urlencode(stripslashes($value2));
     }
     else {
          $value = urlencode(stripslashes($value));
     }
     $req .= "&$key=$value";
 }