<form method="GET" id="getForm">
    <p>First Name <input type="text" name = "firstName" size="20"/></p>
    <p>Last Name <input type="text" name = "lastName" size="20"/></p>
     <button type = "submit" name="submit">Register</button>
</form>

<script>
        var xHRObject = new XMLHttpRequest();
        document.getElementById('getForm').addEventListener('submit', retrieveInformation);
        function retrieveInformation(e)
        {
              e.preventDefault();
              xHRObject.open("GET", "php/register.php", true);
              xHRObject.onreadystatechange = function() {
                   if (xHRObject.readyState == 4 && xHRObject.status == 200)
                       document.getElementById('information').innerHTML = xHRObject.responseText;
              }

              xHRObject.send(null);
        }
</script

if(isset($_GET['submit'])){
    $firstName = $_GET['firstName'];
    $lastName = $_GET['lastName'];

    echo $firstName;
 }