Safe Way to Send POST Request via Javascript

Question

I'm currently sending POST requests to a PHP file of mine via a button with the following function:

function buttonFunction() {
    $.post("http://ipaddress/core/file.php",{username:username, password:pword, coins:coins}, function(data) {
        // Stuff
    });
}

However, I recently found out that if the file process/PHP script is still running (trying to obtain the resulting data/response), and the user refreshes the page, the PHP process would still be running on the server. Also, if the user then decided to click the button again (after refreshing), there would be TWO PHP proccesses running from the same user on the server (that is, if the first one was still running):

Javascript Abort POST Request on Retry/Refresh (HIGH CPU Usage)

However, I came across sending POST data with XMLHttpRequest with Javascript:

Send POST data using XMLHttpRequest

So let's say I send my POST request this way, would it be safe to say when the user refreshes/closes out of the page, the PHP execution ends?

function buttonFunction() {
    var http = new XMLHttpRequest();
    var url = "get_data.php";
    var params = "lorem=ipsum&name=binny";
    http.open("POST", url, true);

    //Send the proper header information along with the request
    http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    http.setRequestHeader("Content-length", params.length);
    http.setRequestHeader("Connection", "close");

    http.onreadystatechange = function() {//Call a function when the state changes.
        if(http.readyState == 4 && http.status == 200) {
            alert(http.responseText);
        }
    }
    http.send(params);
}

However, if this also does not work, how can I fix this issue (multiple scripts running in the background from the same user)? Whether that fix be in the PHP file or in the JavaScript itself, any help would be appreciated.

Edit 1:

Possible Solution?

What if I use XMLHttpRequest and abort the request before the page unloads?

window.onbeforeunload = function(event) {
    http.abort();
};