无法使用PHP处理表单

Question

这是我的welcome_post.php

<?php 

    // include db connect class
    require_once __DIR__ . '/db_connect.php';

    // connecting to db
    $db = new DB_CONNECT();

    $response = array();



    if (isset($_POST['user_email']) && isset($_POST['user_postcode'])){
        $email      = $_POST['user_email'];
        $postcode   = $_POST['user_postcode'];
        // Insert data into mysql
        $result = "INSERT INTO email (Email,Postcode) VALUES ('$email','$postcode')";
        $insert_db = mysqli_query($db->getConnection(),$result);
        }   


?>

这是我的index.html

<form class="form-inline validate signup" action="welcome_post.php" method="post" role="form">
                        <div class="form-group">
                            <input type="email" class="form-control" name="user_email" id="exampleInputEmail1" placeholder="Enter your email address"><br><br>
                            <input type="email" class="form-control" name="user_postcode" id="exampleInputPostcode1" placeholder="Enter your postcode">
                        </div>
                        <br><br>
                        <input type="submit" name="subscribe" value="Get notified when we cover new area!" class="btn btn-theme">
                    </form>

我的index.html里面包含javascript：

$(document).ready( function () {
    $('#wrapper').height($(document).height());
    // I only have one form on the page but you can be more specific if need be.
    var $form = $('form');

    if ( $form.length > 0 ) {
        $('form input[type="submit"]').bind('click', function ( event ) {
            if ( event ) event.preventDefault();
            // validate_input() is a validation function I wrote, you'll have to substitute this with your own.
            if ( $form.validate() ) { register($form); }
        });
    }
});

function appendResult(userText , className, iconClass){
  var resultHTML = "<div class='stretchLeft result "+ className + "'>" + userText + " <span class='fa fa-" + iconClass + "'></span>" + "</div>";
  $('body').append(resultHTML);
  $('.result').delay(10000).fadeOut('1000');
}


function register($form) {
    $.ajax({
        type: $form.attr('method'),
        url: $form.attr('action'),
        data: $form.serialize(),
        cache       : false,
        dataType    : 'json',
        contentType: "application/json; charset=utf-8",
        error       : function(err) { alert("Could not connect to the registration server. Please try again later."); },
        success     : function(data) {
            if (data.result != "success") {
                appendResult('Wrong Email Or You Are Already Registered, Try Again', 'error', 'exclamation');
            } else {
                // It worked, carry on...
                appendResult('Successful, Check Your Email For Confirmation ', 'success', 'check');
            }
        }
    });
}

当我按下提交按钮时，它无法插入数据库并显示javascript警告＆＃34;无法连接到注册服务器。稍后再试。＆＃34;

我试过运行php代码，没有错误。