You must install the mhash library if you are running PHP v5.1.1 or less. Your  PHP version is &lt;i&gt;{version}&lt;/i&gt;.

&lt;b&gt;Please configure the Silent Post URL:&lt;/b&gt;

1. Log into the Merchant Interface at https://secure.authorize.net
2. Click Settings under Account in the main menu on the left
3. Click Silent Post URL in the Transaction Format Settings section
4. Enter this as the secondary URL and save the form:

{url}