AJAX PHP数据库不工作没有错误

I am new to AJAX and am trying to insert form data into a database using ajax and php. It is simply not working, I had errors to start off but now there are no more errors and the database doesn't update and there is no response text. Please Help!

AJAX

    function submit(){
        var vid = <?php echo $user['vid'] ?>;
        var type = document.getElementById("type").value;
        var rules = document.getElementById("rules").value;
        var comments = document.getElementById("comments").value;
        var xhttp;
        if (window.XMLHttpRequest) {
            xhttp = new XMLHttpRequest();
        } else {
            // code for IE6, IE5
            xhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xhttp.onreadystatechange = function() {
            if (xhttp.readyState == 4 && xhttp.status == 200) {
                document.getElementById("page").innerHTML = xhttp.responseText;
            }
        }
        xhttp.open("POST", "/content/training/request/submit.php", false);
        xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        xhttp.send("vid="+vid+"&type="+type+"&rules="+rules+"&comments="+comments);
    }
</script>

PHP FILE AJAX IS CALLING:

<?php
require("http://".$_SERVER["HTTP_HOST"]."/config/db.php");
$stmt = $db->prepare("INSERT INTO trainingRequests (vid, type, rules, comments, timeSubmitted) VALUES (:vid,:type,:rules,:comments,:timeSubmitted)"); 
$stmt->bindParam(':vid', $vid);
$stmt->bindParam(':type', $type);
$stmt->bindParam(':rules', $rules);
$stmt->bindParam(':comments', $comments);
$stmt->bindParam(':timeSubmitted', $time);

$vid = $_POST["vid"];
$type = $_POST["type"];
$rules = $_POST["rules"];
$comments = $_POST["comments"];
$time = time();

$stmt->execute();

echo "Submitted!";