如何从jtml页面获取信息用javascript到php

How do I get information from html page with javascript to php? I hardcode my userid and password but it cant work...

Javascript:

<script type="text/javascript">

    var user = ["jack", "james", "lisa"];
    var pass = ["jack123", "james123", "lisa123"];

    function login() {
        var uid = document.getElementById("uid").value;
        var pin = document.getElementById("PIN").value;
            if (uid == user[0] && pin == pass[0] || uid == user[1] && pin == pass[1] || uid == user[2] && pin == pass[2]){
                alert("Log in Successful");
                window.location.href = "Welcom(withdrawal).php" ;
            }
            else{
                alert("Your User ID and PIN do not match!");
                return false;
            }
    }

</script>

Form:

<section class="right" >
    <table bgcolor="#FFFFFF" border="0" >
    <form method="post" onsubmit="login();return false;" >
        <caption id="sidehead">Finger-Print banking</caption>
        <tr>
            <td>
            <label for="uid" >User ID :</label>
            <input class="row2" type="text" tabindex="1" maxlength="20" size="20" name="uid" id="uid">
            </td>
        </tr>   
        <tr>
            <td>
            <label for="PIN" >PIN :</label>
            <input class="row2" type="password" onkeyup="keyUp(event)" onkeydown="return onlyNumerics(event)" tabindex="2" maxlength="9" size="20" name="PIN" id="PIN" autocomplete="off">
            </td>
        </tr>
        <tr>
            <td class="m">
            <button id="side2">Login</button>
            <button type="reset" id="side1">Reset</button>
            </td>
        </tr>
        </form>
    </table>
</section>

PHP:

<?php
    $userid = $_POST['uid'];

    echo ($userid);
?>

I did this but nothing come up, it says uid is undefined. what happen?