php ช่วยหน่อยครับ เรื่องโค้ด check สถานะ PayPal payment

<form id="formPaypal" name="formPaypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_ext-enter">
<input type="hidden" name="redirect_cmd" value="_xclick">
<input type="hidden" name="business" value="EMAIL_ADDRESS">
<input type="hidden" name="item_name" value="ITEM_NAME">
<input type="hidden" name="upload" value="1">
<input type="hidden" name="amount" value="PRICE">
<input type="hidden" name="currency_code" value="EUR">
<input type="hidden" name="return" value="DOMAIN/success.php">
<input type="hidden" name="cancel_return" value="DOMAIN/cancel.php">
<input type="hidden" name="invoice" value="INVOICE_NUMBER">
<input type="hidden" name="rm" value="2">
<input type="submit" value="Submit">
</form>

<?php
if ($_POST["payment_status"]=="Completed") {
   //--PAYMENT SUCCESSFUL
}
?>