Increment invoice number on html form submit in WordPress – Phpmyadmin

I have been searching for hours and I do not understand MySQL so I am wondering if it is possible to do this another way. I have a custom html payment form that submits invoice number, amount, and name to a payment processor. I want to be able to add in a transaction number that starts at 30200 and auto increments by one every time the form is submitted to the processor. I wrote a simple increment function:

var i = 30200;
function buttonClick() {
    document.getElementById('transNum').value = ++i;
}

The code passes the incremented variable correctly but doesn’t store the new value anywhere and I do not know how to store it. Every time the form is submitted the transaction number gets set to 30201. Is there a way to store it in an external file and call the last number stored? Like I said I do not know MySQL so storing it in the database is beyond my understanding. There has to be an easy understandable way for me to accomplish this but I cannot find a solution.

EDIT ALMOST THERE (I HOPE)

After a lot of research I am trying to use mySQL to accomplish this task.

So I followed a YouTube video and used phpMyAdmin to create a table called refNum and then created the auto incrementing variable refNumber and set it to 30300.

I created a php file and uploaded it to the server called reference.php with this code inside it:

<?php 
function get_refNum($ref){
global $wpdb;
$results = $wpdb->get_results("SELECT refNumber FROM refNum");
return $results;
};
?>

Then in my form I wrote this code to call the page and get the results from the database:

 function getTransNumber(){
 $.ajax({
 url : '/reference.php',
 method : 'post',
 data : {$results},
 });
 $results = document.getElementById('invoiceNum').value;
 }
 window.onload = getTransNumber;

My problem is now I am getting an error Uncaught reference error: $results is not defined in my ajax call under data. Which means either I am not getting the data correctly in my php code or I am not passing the data. Can someone please point me in the right direction and tell me what I did wrong here?