my first ipn

Hi
I coded a simple ipn by which one can pay any amount he/she wishes to my account. lease review it if it is including any bugs. there is no fixed fees for products, i just need to know what amount has been paid and save it.
form:

<form method="post" action="https://www.paypal.com/cgi-bin/webscr"  >
<p align=center>
Your Username: <input type=text name=username readonly=readonly value="<?php echo $username ;?>" ><br><br>
Amount you want to deposit: <input type=text readonly=readonly value="<?php echo $amount ;?>"  size=10 name=amount>USD
<br><br>
<input type=submit value="Confirm" name="confirm" >
<br><br>Use your browser's Back button to edit the form
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="return" value="http://domain">
<input type="hidden" name="notify_url" value="http://domain/ipn.php">
<input type="hidden" name="cancel_return" value="http://domain">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="bn" value="PP-BuyNowBF">
<input type="hidden" name="business" value="myid@gmail.com">
<input type="hidden" name="mc_gross" value="<?=$amount;?>">
</form>

HTML:

the notify_url:

<?php
$username=$_POST['username'];
$business=$_POST['business']; 
$invoice=$_POST['invoice']; 
$memo=$_POST['memo'];
$tax=$_POST['tax'];
$mc_gross=$_POST['mc_gross'];
$mc_currency=$_POST['mc_currency']; 
$payment_status=$_POST['payment_status'];
$pending_reason=$_POST['pending_reason']; 
$reason_code=$_POST['reason_code']; 
$payment_date=$_POST['payment_date']; 
$txn_id=$_POST['txn_id'];
$first_name=$_POST['first_name']; 
$last_name=$_POST['last_name']; 
$address_name=$_POST['address_name']; 
$address_street=$_POST['address_street']; 
$address_city=$_POST['address_city']; 
$address_state=$_POST['address_state']; 
$address_zip=$_POST['address_zip']; 
$address_country=$_POST['address_country']; 
$address_status=$_POST['address_status']; 
$payer_email=$_POST['payer_email']; 
$payer_id=$_POST['payer_id']; 
$payer_status=$_POST['payer_status']; 

if($payment_status!="completed" || $business!="me@email" || $mc_currency!="USD")
echo "The payment was not successful";
else{
//payment was successful. save it in database
$sql="INSERT INTO table  ";
mysql_query($sql);
echo "Thank you. You have paid successfully";

}

?>

PHP:

 

Why you need anyone for testing?
You can do the testing on your own using sandbox model of paypal.

Paypal Sandbox

Hope this helps..