Update PHP based Paypal Integration to new methods

I have been using a slightly modified Paypal Payment Gateway script from the website below for a little while now

https://phppot.com/php/paypal-payment-gateway-integration-in-php/

It has been working great, but the over a period of 48 hours i started to get infrequent reports of users who have paid (I can see their payment in PayPal) but don’t appear to have their payment recorded into my MySQL database due to issues with the script getting payment verification from PayPal. Because the transactions don’t get recorded in the database, the triggers don’t function to add credits to their account and i have unhappy users. In short, the system has become unreliable and i have pulled it from the server.

I have checked the server logs and i do see this error.

2254443#0: *102962 FastCGI sent in stderr: "ww/vhosts/dankdronedownloader.com/httpdocs/DDD2/payment_processor/notify.php on line 43PHP message: PHP Warning: Undefined variable $get_magic_quotes_exists in /var/www/vhosts/dankdronedownloader.com/httpdocs/DDD2/payment_processor/notify.php on line 43

I have been advised by the script creator that I could add one line of code to work around this, however I do keep getting emails from PayPal telling me "its time to update your integration" which links me to this website below, so I want to take this opportunity to upgrade the integration to the new standard as the current method will eventually be retired.

https://developer.paypal.com/docs/checkout/standard/upgrade-integration/?utm_source=paypal&utm_medium=email&utm&_ga=2.207277395.1389730440.1696410198-2139970517.1695896604



Goals
Attached in a zip file (renamed as a .txt) is my current (sanitised) code structure so you can load this up and see how it works. In short, I want nothing visible to change, the page has to look and function how it does, however the PayPal payment side of things needs to be updated to use the new payments system.

I am happy to provide sandbox API keys/whatever you need to accomplish this.

The payments MUST be recorded in the MySQL database 100% of the time and ideally either alert me or the user if this fails. I want 0% chance of a user paying, but things not being recorded.

An important factor is a value on index.php called ‘id’. For me to associate payments to a user of the app, when users click on a button in the associated app (out of scope for this project) they are taken to a URL like “payment_processor/index.php?id=ID_Goes_Here”. This is stored as a PHP variable on line 165 of index.php and is used thought the script, this MUST get recorded in the database to allow my other triggers and functions to detect this and assign credits. How this works is also out-of -scope, just assume that if you place valid data in the database, the triggers deal with the credit association

The database structure also cannot be altered as the same database table is used by Stripe and is integral to the app function, changes would require additional work in many other areas incurring other time and cost.

Any new/additional code should be well commented to allow for future modifications.

Credits are sold in quantities of 1,5,15 or 30, it needs to at least allow for these values with a clear way to allow alternate values to be added in the future (maybe even freeform where the user could purchase 7 credits for example).

I am looking to start this project immediatly.

NOTE: The attached source code is a zip file, but PPH doesn't allow these. Please rename the file to a .zip and extract it.