Payment with PayPal hangs inside the notify_url page - php

Question

I have this page to pay with paypal, and receive the notification in "notify_url". The purchase goes well but fails to receive the notification. Everything is fine until it reaches these two lines: $valid_txnid = check_txnid ($data ['txn_id']);$valid_price = check_price ($data ['payment_amount'], $data ['item_number']);
where the program hangs. (I know it because I have sent emails from various points of the program until I get there that I do not receive anything).
And also I don't know where the check_txnid () and check_price () functions come from, which I don't see defined anywhere. The code is taken from this other post: How to get transaction details in notify_url page in paypal
index.php
<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post" style="padding: 0; margin: 0;">
<input type="hidden" name="cmd" value="_xclick" />
<input type="hidden" name="business" value="sb-pwrdu1932120#business.example.com" />
<input type="hidden" name="quantity" value="1" />
<input type="hidden" name="item_name" value="MiArticulo" />
<input type="hidden" name="item_number" value="1" />
<input type="hidden" name="amount" value="1" />
<input type="hidden" value="19" name="invoice">
<input type="hidden" name="notify_url" value="http://infrangible-discoun.000webhostapp.com/pruebaPaypal/notifyUrl.php">
<input type="hidden" name="currency_code" value="EUR" />
<input type="hidden" name="rm" value="2" >
<input type="hidden" name="return" value="http://infrangible-discoun.000webhostapp.com/paypal4/newfile1.php">
<input type="image" border="0" name="paypal" src="images/btn_paypal_nl.gif" onClick=""/>
</form>
notifyUrl.php
<?php
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;
use PHPMailer\PHPMailer\Exception;
require("conexion.php");
require 'phpmailer/Exception.php';
require 'phpmailer/PHPMailer.php';
require 'phpmailer/SMTP.php';
// Response from Paypal
// read the post from PayPal system and add 'cmd'
$req = 'cmd=_notify-validate';
foreach ($_POST as $key => $value) {
$value = urlencode(stripslashes($value));
//$value = urlencode($value);
$value = preg_replace('/(.*[^%^0^D])(%0A)(.*)/i','${1}%0D%0A${3}',$value);// IPN fix
$req .= "&$key=$value";
}
// assign posted variables to local variables
$data['item_name'] = $_POST['item_name'];
$data['item_number'] = $_POST['item_number'];
$data['payment_status'] = $_POST['payment_status'];
$data['payment_amount'] = $_POST['mc_gross'];
$data['payment_currency'] = $_POST['mc_currency'];
$data['txn_id'] = $_POST['txn_id'];
$data['receiver_email'] = $_POST['receiver_email'];
$data['payer_email'] = $_POST['payer_email'];
$data['custom'] = $_POST['custom'];
$data['invoice'] = $_POST['invoice'];
$data['paypallog'] = $req;
// post back to PayPal system to validate
$header = "POST /cgi-bin/webscr HTTP/1.0\r\n";
$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Content-Length: " . strlen($req) . "\r\n\r\n";
$fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);
if (!$fp) {
// HTTP ERROR
} else {
fputs ($fp, $header . $req);
while (!feof($fp)) {
////mail('atiftariq80#gmail.com','Step 9','Step 9');
$res = fgets ($fp, 1024);
if (true || strcmp($res, "VERIFIED") == 0) {
////mail('atiftariq80#gmail.com','PAYMENT VALID','PAYMENT VALID');
// Validate payment (Check unique txnid & correct price)
$valid_txnid = check_txnid($data['txn_id']);
$valid_price = check_price($data['payment_amount'], $data['item_number']);
// PAYMENT VALIDATED & VERIFIED!
if($valid_txnid && $valid_price){ //doesn't pass this if.
//----------------- INSERT RECORDS TO DATABASE-------------------------------------
$name=$_POST['item_name'];
mysqli_query($db,"insert into ventas value(0,'$name')");
if ($data['invoice']=='basic') {
$price = 39;
} else {
$price = 159;
}
$this->user_model->update_user(
array(
'id' => $data['custom'],
'user_status' => 1,
'payment_date' => date("Y-m-d H:i:s",time()),
'next_payment_date' => date('Y-m-d', strtotime('+32 days')),
'user_package' => $data['invoice'],
'package_price' => $price
)
);
$data2 = array('id' => '',
'txn_id' => $data['txn_id'],
'amount' => $data['payment_amount'],
'mode ' => $data['payment_status'],
'paypal_log' => $data['paypallog'],
'user_id' => $data['custom'],
'created_at' => date('Y-m-d H:i:s',time())
);
$this->db->insert('tbl_paypal_log', $data2);
//----------------- INSERT RECORDS TO DATABASE-------------------------------------
}else{
// Payment made but data has been changed
// E-mail admin or alert user
}
} elseif ($res=='INVALID') {
// PAYMENT INVALID & INVESTIGATE MANUALY!
// E-mail admin or alert user
////mail('atiftariq80#gmail.com','PAYMENT INVALID AND INVESTIGATE MANUALY','PAYMENT INVALID AND INVESTIGATE MANUALY');
}
}
fclose ($fp);
}
?>

Answer 1

I don't know where the check_txnid () and check_price () functions come from, which I don't see defined anywhere.
That seems to be your problem. Don't call functions that don't exist.
If you want to to check the transaction ID and the price are unique and valid for what you are offering, create actual functions that actually do this.
You could for example use 'custom' or 'invoice' to match against an existing order record in your database and validate the price corresponds, if you provided such a unique number to PayPal as part of the transaction.