$(document).ready(function(){
    $('#Submit').click(function() {
        $.ajax({
            type : 'POST',
            url : './helpers/checkuser.php',
            data: {
                action: "checklogin",
                user : $('#username').val(),
                pass   : $('#password').val()
            },
            success:function (data) {
                if (data == "Valid Login")
                    window.location.replace("../login.php");
                else
                    alert(data);
            }
        });
    });
});

// QUERY THE DB
$stmt = $link->prepare('SELECT * FROM users WHERE username = '$username' AND password = '$password'');
$stmt->bind_param('s', $search);
$stmt->execute();
$result = $stmt->get_result();

if(mysqli_num_rows($result) > 0)
{
     echo "Valid Login";
}