Как исправить функцию добавления?

Я делаю проект на основе php. И я хочу добавить данные в базу данных. Я сделал это с помощью php, PDO. И я создал код с ООС. Но на главной HTML-странице есть часть if else. В этой части работает другая часть. Но я даже не нажал кнопку вставки. Когда страница загружается, выполняется другая часть. Я посмотрел видео, и этот код работает отлично. Вот код..

Insert.php - геттеры, сеттеры, SQL-запрос и т. д.

    class Stock
    {
        protected $ItemNo;
        protected $ItemName;
        protected $brand;
        protected $qty;
        protected $Description;
        protected $ItemDate;
        protected $SupplierName;
        protected $SupplierMail;
        private $tableName = 'StockDetails';
        private $dbconn;

        function setItemNo($ItemNo) { $this->ItemNo = $ItemNo; }
        function getItemNo() { return $this->ItemNo; }
        function setItemName($ItemName) { $this->ItemName = $ItemName; }
        function getItemName() { return $this->ItemName; }
        function setBrand($brand) { $this->brand = $brand; }
        function getBrand() { return $this->brand; }
        function setQty($qty) { $this->qty = $qty; }
        function getQty() { return $this->qty; }
        function setDescription($Description) { $this->Description = $Description; }
        function getDescription() { return $this->Description; }
        function setItemDate($ItemDate) { $this->ItemDate = $ItemDate; }
        function getItemDate() { return $this->ItemDate; }
        function setSupplierName($SupplierName) { $this->SupplierName = $SupplierName; }
        function getSupplierName() { return $this->SupplierName; }
        function setSupplierMail($SupplierMail) { $this->SupplierMail = $SupplierMail; }
        function getSupplierMail() { return $this->SupplierMail; }

        public function __construct()
        {
            require_once ('Database.php');
            $db = new Database();
            $this->dbconn = $db->connect();

        }
        public function insert(){

            $sql = "INSERT INTO $this->tableName VALUES (:ItemNo, :ItemName, :brand, :qty, :Description, :ItemDate, :SupplierName, :SupplierMail)";
            $stmt = $this->dbconn->prepare($sql);

            $stmt->bindParam(':ItemNo', $this->ItemNo);
            $stmt->bindParam(':ItemName', $this->ItemName);
            $stmt->bindParam(':brand', $this->brand);
            $stmt->bindParam(':qty', $this->qty);
            $stmt->bindParam(':Description', $this->Description);
            $stmt->bindParam(':ItemDate', $this->ItemDate);
            $stmt->bindParam(':SupplierName', $this->SupplierName);
            $stmt->bindParam(':SupplierMail', $this->SupplierMail);

            if ($stmt->execute()){

                return true;

            }else{

                return false;
            }}}  
?>

Action.php — действие формы на AddItem.php

    require_once ('Insert.php');

    class action{

        function __construct(){

            switch ($_POST['submit']) {
                case 'insert':
                    $obInsert = new Stock;

                    $obInsert->setItemNo($_POST['ItemNo']);
                    $obInsert->setItemName($_POST['ItemName']);
                    $obInsert->setBrand($_POST['brand']);
                    $obInsert->setQty($_POST['qty']);
                    $obInsert->setDescription($_POST['Description']);
                    $obInsert->setItemDate(date('Y-m-d H:i:s'));
                    $obInsert->setSupplierName($_POST['SupplierName']);
                    $obInsert->setSupplierMail($_POST['SupplierMail']);

                    if ($obInsert->insert()) {

                        header('location: AddItem.php?insert=1');

                    } else{

                        header('location: AddItem.php?insert=0');
                    }

                    break;      
                default:
                    header('location: AddItem.php');
                    break;
            }
        }
    }

    if (isset($_POST['submit'])){

        $object = new action;
    }

AddItem.php

Чтобы получить бренд и ItemName из базы данных. Бренд меняется в зависимости от ItemName.

<?php
require_once ('GetBrand.php');
$ItemName = LoadItemName();

?>

   $(document).ready(function(){
        $("#ItemName").change(function(){
          var aid = $("#ItemName").val();
          $.ajax({
             url: 'GetBrand.php',
             method: 'post',
             data: 'aid=' + aid
           }).done(function(brand){
             console.info(brand);
             brand = JSON.parse(brand);
             $('#brand').empty();
             brand.forEach(function(bnamee){
             $('#brand').append('<option>' + bnamee.brand + '</option>')
             })
           })
         })
        })

Форма

<form name = "form0" method = "post" action = "Action.php">
    <div class = "form-group">
         <label for = "text">Item No</label>
         <input type = "text" name = "ItemNo" id = "ItemNo" placeholder = "Item No" required>
     </div>
     <form name = "form1" action = "AddItem.php" method = "post">
          <div class = "form-group">
              <label for = "text">Item Name</label>
              <table>
                 <tr>
                    <td><select name = "ItemName" id = "ItemName" required>
                    <option value = "" disabled = "" selected>Select Name</option>
                    <?php foreach($ItemName as $iname)
                        echo "<option id='".$iname['ItemNo']."' value='".$iname['ItemNo']."'>".$iname['ItemName']."</option>";
                    ?>
                  </select></td>
                  <td><label for = "text">Add Item Name : </label></td>
                  <td><input type = "text" name = "name" id = "name"> </td>
                  <td><button>Add</button></td>
                </tr>
             </table>
         </div>
         <div class = "form-group">
             <label for = "text">Brand Name</label>
             <table>
                <tr>
                   <td><select name = "brand" id = "brand" required>
                   <option value = "">Select Brand</option>
                   </select></td>
                   <td<label for = "text">Add Brand : </label></td>
                   <td><input type = "text" name = "Bname" id = "Bname"  class = "form-control" > </td>
                   <td><button>Add</button></td>
               </tr>
           </table>
        </div>
     </form>
     <div class = "form-group">
        <label for = "text">Quantity</label>
        <input type = "text" name = "qty" id = "qty" placeholder = "Quantity" required>
     </div>
     <div class = "form-group">
        <label for = "text">Item Description</label>
        <input type = "text" name = "Description" id = "Description" placeholder = "Description" required>
      </div>
      <div class = "form-group">
          <label for = "text">Date</label>
          <input type = "date" name = "ItemDate" id = "ItemDate" required>
      </div>
      <div class = "form-group">
          <label for = "text">Supplier Name</label>
          <input type = "text" name = "SupplierName" id = "SupplierName"  placeholder = "Supplier Name" required>
      </div>
      <div class = "form-group">
          <label for = "text">Supplier Mail</label>
          <input type = "text" name = "SupplierMail" id = "SupplierMail" placeholder = "Supplier Mail" required>
       </div><br/>
       <div class = "form-group">
           <button id = "submit" name = "submit" value = "insert">Insert</button>
           <?php 
              if (isset($_GET['insert']) && ($_GET['insert'] == 1)){
                echo "Inserted Successfully ";
              }else // This else part is running when the page is loading 
                echo "Ooops..! Something went wrong.";
              }
           ?>
       </div>
     </form>