Firebase server side validation using PHP scripts - php

Question

I have 10 ios apps with subscription in app purchases. I need one subscription purchase to be valid accross the 10 apps. Thus I require server-side receipt validation. The flow is like this: When the customer pays for the subscription, the receipt is sent to the Firebase DB and from there, I require a PHP script that takes in the receipt data . as input and sends a 'POST' request to the App store. The App store would then validate the receipt and return a JSON object back. We then overwrite the old receipt with the latest copy. Also, whenever the user logs in to any of the apps, we repeat this process and update the receipt to make sure the subscription of the user is still valid. How do I do this in Firebase? If I cannot, can you please suggest any alternatives?
I have seen on some forums that Firebase 'cloud functions' might be able to such things. However, I am not exactly sure. Also, I am not quite adamant on using PHP. If I am able to achieve the same outcome using a different scripting language, I would be very happy.
Thanks.
Here is a sample of the PHP script:
<?php
function getReceiptData($receipt)
{
$fh = fopen('showme.txt',w);
fwrite($fh,$receipt);
fclose($fh);
$endpoint = 'https://sandbox.itunes.apple.com/verifyReceipt';
$ch = curl_init($endpoint);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $receipt);
$response = curl_exec($ch);
$errno = curl_errno($ch);
$errmsg = curl_error($ch);
curl_close($ch);
$msg = $response.' - '.$errno.' - '.$errmsg;
echo $response;
}
foreach ($_POST as $key=>$value){
$newcontent .= $key.' '.$value;
}
$new = trim($newcontent);
$new = trim($newcontent);
$new = str_replace('_','+',$new);
$new = str_replace(' =','==',$new);
if (substr_count($new,'=') == 0){
if (strpos('=',$new) === false){
$new .= '=';
}
}
$new = '{"receipt-data":"'.$new.'","password":"<INSERT YOUR IN-APP PURCHASE SHARED SECRET HERE>"}';
$info = getReceiptData($new);
?>