<form action="https://somesite.com" method="POST" id="form_one">

    <fieldset class="info">
    <legend>Personal Information</legend>
    <div>
        <label for="name">name</label>
        <input type="text" name="param1" id="name">
    </div>
    <div>
        <label for="email">e-mail</label>
        <input type="email" name="param2" id="email" required><br>
    </div>
    <div>
        <input type="submit" class="submit">
    </div>
    </fieldset>

    <fieldset class="output">
        <legend>Response</legend>
        <div id="message"></div>

        <div>
            <label for="out-name" class="hidden" id="label-out-name">
                Your name
            </label>
            <output for="name" id="out-name"></output>
        </div>
        <div>
            <label for="out-email" class="hidden" id="label-out-email">
                Your email address
            </label>
            <output for="email" id="out-email"></output>
        </div>
        <div>
            <label for="submitted-params" class="hidden" id="label-submitted-params">
                Submitted params (POST)
            </label><br>
            <output for="name email" id="submitted-params"></output>
        </div>
    </fieldset>
</form>

document.getElementById('form_one').addEventListener('submit', function (event) {
    event.preventDefault();
    var param1 = document.getElementById('form_one')[1].value;
    var param2 = document.getElementById('form_one')[2].value;
    var params = generateParams('param1', param1, 'param2', param2);
    console.log("%c %s", "color:green", params)
    document.getElementById("submitted-params").value = params;
    document.getElementById("label-submitted-params").style.visibility = 'visible';
    makePostRequest("https://php.tomgdow.com", params, ajaxCallMe());
});

function makePostRequest(url, params, fn) {
    let oReq = new XMLHttpRequest();
    oReq.addEventListener("load", fn);
    oReq.open('POST', url);
    oReq.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    // oReq.setRequestHeader("Content-length", params.length);
    // oReq.setRequestHeader("Connection", "close");
    oReq.send(params);
}

function ajaxCallMe() {
    return function ajax() {
        let response = this.responseText;
        let message = "Greetings from the PHP Server (POST Method)";
        let responseArray = response.split('&');
        document.getElementById('out-name').value = responseArray[0];
        document.getElementById('out-email').value = responseArray[1];
        document.getElementById('message').innerHTML = message;
        console.log("%c %s", "color:red", response);
        console.log("%c %s", "color:blue", responseArray[0]);
        console.log("%c %s", "color:blue", responseArray[1]);
        console.log("%c %s", "color:blue", responseArray[2]);
        document.getElementById("label-out-name").style.visibility = "visible";
        document.getElementById("label-out-email").style.visibility = "visible";
    };
}
        
function generateParams(...args) {
    var tmp = "";
    if (args.length < 3) { tmp += `${args[0]}=${args[1]}`; } else {
        tmp += `${args[0]}=${args[1]}`;
        for (let i = 2; i <
            args.length; i += 2) { tmp += `&${args[i]}=${args[i + 1]}`; }
    }
    return tmp;
} 

// index.php (server)

<?php
  echo  
     htmlspecialchars($_POST["param1"])."&"
     .htmlspecialchars($_POST["param2"])."&"
     .htmlspecialchars($_SERVER['HTTP_HOST']);
?>