StackOverflow question

<?php
if(isset($_GET['a'])){
    sleep(5);
    die;
}
?>
<script type="text/javascript">
function request(){
    var ao = new XMLHttpRequest();
    ao.onreadystatechange = function(){
        var e = document.getElementById('status');
        switch(ao.readyState){
            case 1:
                e.innerHTML = '(' + (new Date().getSeconds()) + ') Connection opened.';
                break;
            case 2:
                e.innerHTML += '<br />(' + (new Date().getSeconds()) + ') Sending request...';
                break;
            case 3:
                e.innerHTML += ' done.<br />(' + (new Date().getSeconds()) + ') Processing by server...';
                break;
            case 4:
                e.innerHTML += ' done.<br />(' + (new Date().getSeconds()) + ') AJAX request complete!';
                document.getElementById('btn').disabled = false;
                break;
        }
    }
    ao.open('GET', 'wait.php?a', true);
    ao.send();
}
</script>
<div id="status"></div>
<input type="submit" value="Send request" onclick="request();this.disabled=true" id="btn" />