How to integrate Paypal IPN for recurring payments?

I have been using Micah Carrick's PAYPAL IPN class till now for

web_accept

but now one of the Client wants to integrate the Recurring method into it.I tried using the same stuff but unfortunately was not so successful this time. I am trying to connect to the Paypal recurring method.Following is the code by which I trying to get to it;

$paypalObj = new paypal_class();



$itemName = "My Product";

$itemNumber = $itemName . " - Premium ($amount$)";



$paypalObj->paypal_url = 'https://www.sandbox.paypal.com/cgi-bin/webscr';// testing paypal url

//$paypalObj->paypal_url = 'https://www.paypal.com/cgi-bin/webscr';// Live paypal url



$paypalObj->add_field('cmd','_xclick-subscriptions');

$paypalObj->add_field('txn_type', "recurring_payment");

$paypalObj->add_field('product_name', "My Product Subscription - Monthly( $$amount )");

$paypalObj->add_field('desc', "My Product Subscription - Monthly( $$amount )");



$paypalObj->add_field('business', "test@yahoo.com");



$paypalObj->add_field('return', "http://".$serverName . "/buy-now.php");

$paypalObj->add_field('cancel_return', "http://".$serverName. "/return.php?action=cancel");

$paypalObj->add_field('notify_url', "http://" . $serverName . "/return.php?action=ipn");



$paypalObj->add_field('src', "1");

$paypalObj->add_field('sra', "1");

$paypalObj->add_field('a3', "$amount");

$paypalObj->add_field('t3', "M");

$paypalObj->add_field('p3', "1");

//$paypalObj->add_field('no_note', "1");

$paypalObj->add_field('currency_code', "USD");



$paypalObj->add_field('first_name', $firstName);

$paypalObj->add_field('last_name', $lastName);



$paypalObj->submit_paypal_post();

Code lets me to redirect to the Paypal sandbox(As I am still testing),but when I login onto Paypal sandbox it throws me this Error :

The link you have used to enter the PayPal system is invalid. Please review the link and try again.

Source: Tips4all