AJAX，没有收到任何回复

 if(isset($_GET['delete'])) {
$ID = $_GET['delete'];
echo "
<script type='text/javascript'>
    var ajax = new XMLHttpRequest();
    var ajax.open('POST','parser.php', true);
    ajax.setRequestHeader('Content-type','application/x-www-form-urlencoded');
    ajax.onreadystatechange = function () {
        if(ajax.readyState ==4 && ajax.status == 200) {
            alert(ajax.responseText);
        }
        }
        ajax.send('delete=$ID')
    </script>
    ";
}

this is my code, it is supposed to send the id to parser.php

here is parser.php

    <?php

if(isset($_POST['delete'])) {
    echo $_POST['delete'];
}

no response is received from parser.php, can anyone help ? all answers are much appreciated