paypal IPN not sending any notification - php

Question

I have integrated paypal with my website and but it does take the payment but it doesnot send any IPN to my php file which should update my databse....
there is the code I have been using:
to call
session_start();
if($mode=='live')
{
$url=" https://www.paypal.com/cgi-bin/webscr";
$email="nnnnnnn_singh#live.com";
$_SESSION['url']=$url;
$_SESSION['email']=$email;
$_SESSION['subtotal']=$subtotal;
}
else if($mode=='sandbox')
{
$url= "https://www.sandbox.paypal.com/cgi-bin/webscr";
$email="nnnnnnn_1329707350_biz#gmail.com";
$_SESSION['url']=$url;
$_SESSION['email']=$email;
$_SESSION['subtotal']=$subtotal;
}
?>
<form action="<?php echo $url; ?>" method="post">
<input type="hidden" name="cmd" value="_xclick"/>
<input type="hidden" name="business" value="<?php echo $email; ?>"/>
<input type="hidden" name="item_name" value="shopping cart"/>
<input type="hidden" name="currency_code" value="GBP"/>
<input type="hidden" name="amount" value="<?php echo $subtotal;?>"/>
<input type="hidden" name="return" value="<?php echo SITE_URL; ?>"/>
<input type="hidden" name="cancel_return" value="<?php echo SITE_URL."/shoppingCart.php" ?>"/>
<input type="hidden" name="notify_url" value="php url" />
<input type="hidden" name="rm" value="2"/>
<input type="hidden" name="custom" value="<?php echo $customer_ID;?>"/>
<input type="image" src="http://www.paypal.com/en_US/i/btn/x-click-but01.gif" name="submit" alt="Make payments with PayPal - it's fast, free and secure!"/>
</form>
and php code
from php IPN sample code.....

Answer 1

Try:
1. login in your paypal account
2. go to profile --> History --> IPN history --> turn on my IPN
3. insert your IPN listener URL.

Answer 2

What's the value of:
<input type="hidden" name="notify_url" value="php url" />
This shouldn't be on localhost, since PayPal sends POST data from its servers to that URL. So localhost won't work.