(do lots of order processing things here, such as write appropriate things to the database.  Then ship
the transaction to Paypal.

        $postfields ="cmd=".urlencode("ext-enter")."&";
	$postfields.="redirect_cmd=".urlencode("_xclick")."&";
	$postfields.="business=".urlencode($pp_business)."&";
	$postfields.="amount=$total&";
	$postfields.="currency_code=USD&";
	$postfields.="notify_url=".urlencode("$securesite/paypal_notify.php")."&";
	$postfields.="return=".urlencode("$securesite/paypal_return.php")."&";
	$postfields.="cancel_return=".urlencode("$securesite/paypal_cancel.php")."&";
	$postfields.="invoice=".$cart_id." &";
	$postfields.="item_name=".urlencode("Payment for Order Number $cart_id");
	$ch = curl_init();
	curl_setopt ($ch, CURLOPT_POST,true);
	curl_setopt ($ch, CURLOPT_POSTFIELDS, $postfields); 
	//curl_setopt($ch,CURLOPT_POSTFIELDS,"business=$pp_business");
	curl_setopt($ch, CURLOPT_HEADER, false);
	curl_setopt ($ch, CURLOPT_URL,$pp_gateway);
	curl_setopt ($ch, CURLOPT_FORBID_REUSE, false);
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
	$response = curl_exec($ch);
	$response = str_replace("top.location.replace(document.location);"," ",
$response);
	echo $response;
//exit();
	if (curl_errno($ch)) {
   		print curl_error($ch);
	} else {
	    	curl_close($ch);
	}
}

php
// paypal notification URL for instant payments on paypal
// needed to verify transactions and payment status 
require("cartfcns.php");
require_once("config.php");
$cart_id=$_REQUEST['invoice'];
$txn_id=$_REQUEST['txn_id'];
$payment_status=$_REQUEST['payment_status'];
$receiver_email=$_REQUEST['receiver_email'];
mysql_connect("$host","$user","$pass");
mysql_select_db("$database");
$qstr="SELECT * FROM custlist WHERE cart_id='$cart_id'";
$result=mysql_query($qstr);
$row = mysql_fetch_array($result,MYSQL_ASSOC);
$_REQUEST['cmd']="_notify-validate";
$usearray = array();
foreach($_REQUEST as $key=>$value) {
	 $usearray[]=var_export($key,true).'='.urlencode(var_export($value,true));
}
$outstr=implode('&',$usearray);
$outstr=str_replace("%27","",$outstr);
$outstr=str_replace("'","",$outstr);
$session = curl_init($pp_gateway);
curl_setopt ($session, CURLOPT_POST, true);
curl_setopt ($session, CURLOPT_POSTFIELDS, $outstr); 
curl_setopt($session, CURLOPT_HEADER, false);
curl_setopt($session, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($session);
curl_close($session);
//$response="VERIFIED";
if($response == "VERIFIED"){
	if($row['cart_id']=="" or !($receiver_email == $pp_business) or !($row['amount'] == $_REQUEST['mc_gross']) or !($_REQUEST['mc_currency']=='USD')){
		$fp=fopen("PPcommandline.txt","a");
		fputs($fp,$outstr);
		fclose($fp);
		die();
	}
	$doadownload = false;
	$qstr = "UPDATE custlist SET verification='$payment_status',txn_id='$txn_id' WHERE cart_id='$cart_id'";
 	$result=mysql_query($qstr);
	if($payment_status == "Completed"){
		$email=$row['email'];
		$custname=$row['name'];
		$message="Dear $custname;\n\nYour Paypal payment has been approved.\n";
		$qstr = "SELECT * from purchlist WHERE cart_id='$cart_id'";
		$result=mysql_query($qstr);
		$row = mysql_fetch_array($result,MYSQL_ASSOC);
		if($row) {
			$doadownload=true;
			$ii=1;
			$useurl = array();
			$prodlist = array();
			$regno = array();
			while($row){
				$useurl[$ii]=$row['URL'];
				$prodlist[$ii]=$row['code_no'];
				$regno[$ii]=Make_Registration($prodlist[$ii]);
				$ii=$ii+1;
				$row = mysql_fetch_array($result,MYSQL_ASSOC);
			}
			$ii=$ii-1;
		}//end of if($row)
		if($doadownload) {
			$message.="You may now download the products you have ordered by clicking on the enclosed links.  If your products require a keyfile, you will receive that by a separate email.\n\nPlease complete your downloads within 72 hours; beyond that time the enclosed links may not be valid.";
			$jj=1;
			while($jj <= $ii){
				$qstr="SELECT keyfile,item FROM products WHERE code_no='".$prodlist[$jj]."'"; 
				$result=mysql_query($qstr);
				$row = mysql_fetch_array($result,MYSQL_ASSOC);
				$ourkey=$row['keyfile'];
				if ($ourkey <> '' and $ourkey <> 'RegOnly'){
					include "mail_attachment.php";
					$ourkey=$files_path.$ourkey;
					$keymsg="Do not reply to this email; the mailbox is not monitored by a human and the reply will be discarded.\n  Thank you for your order.  This email contains as an attachment a keyfile that is critically important to the operation of The Property Manager.\n  After installing your download, unzip this keyfile directly into your installation folder, which by default will be C:\ThePropertyManager.\n  When you first start the program, it will require you to enter your registration number.  That number is ".$regno[$jj].", which is case sensitive and must be entered exactly as written here.\nIf you have any troubles, contact support@softwareforlandlords.com";
					$subject =   "This is your program keyfile and registration";  
					mail_attachment ($receipt , $email, $subject, $keymsg, $ourkey);
				}// end if $ourkey
				$message.="<a href=\"$securesite/dl_center/index.php?".$useurl[$jj]."\" style=\"color:blue;text-decoration:underline\">Download ".$row['item']."</a>\n";
				if(!($regno[$jj]=="")){
					$message.="<P ALIGN=CENTER><FONT FACE=\"Times New Roman\"><FONT SIZE=3>Your program registration code is ".$regno[$jj].".  Please make a note of it because when you first start the program, you will have to enter it.\n\n";
				}
				$jj=$jj+1;
			}//end while jj <= ii
			$message.="If you have any problems downloading or installing your software, please contact support@softwareforlandlords.com";
		} else {
			$message.="Your order will be shipped within the next business day.\n\n";
		}//end if doadownload
		$mtitle="Paypal Approval of Online Order $cart_id";
		mail("$receipt,$email","$mtitle", "$message", "From:$receipt");
	}// end if payment_status is COMPLETED or PROCESSED
} //end if response=VERIFIED
?>

<?php
// possible payment status values, per PayPal
//Canceled-Reversal
//Completed
//Denied
//Expired
//Failed
//In-Progress
//Partially-Refunded
//Pending
//Processed
//Refunded
//Reversed
//Voided 
if($_REQUEST['st']=="Pending") {
	$result_string = "Thank you for your payment.  Your order will be shipped as soon as your PayPal payment is cleared.";
} else if($_REQUEST['st']=="Denied" or $_REQUEST['st']=="Failed") {
	$result_string = "Your PayPal payment did not go through.  Please contact Just So Software to make other payment arrangements.";
} else if($_REQUEST['st']=="Completed" or $_REQUEST['st']=="Processed") {
	$result_string = "Thank you for your order.  It will be shipped promptly.  If you have requested a download, an email has been sent to you with the download links to follow.";
} else {
	$result_string = "There has been an unknown error in your paypal payment.  The response from Paypal was: ".$_REQUEST['st'].". Please contact Just So Software with this information.";
}
?>
<html>

<head>

<title>Completion form</title>

<style>

A:Link {color:000000;text-decoration:none;}

A:Visited {color:000000;text-decoration:none;}

A:Hover {color:F70404;text-decoration:underline;}

</style>

</head>

<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#660101" VLINK="#660101"
ALINK="#F70404">
<CENTER><?php echo $result_string;?></CENTER>
</BODY>
</HTML>

<html>

<head>

<title>Completion form</title>

<style>

A:Link {color:000000;text-decoration:none;}

A:Visited {color:000000;text-decoration:none;}

A:Hover {color:F70404;text-decoration:underline;}

</style>

</head>

<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#660101" VLINK="#660101"
ALINK="#F70404">
<CENTER>Your payment was canceled.</CENTER>
</BODY>
</HTML>