可捕获的致命错误：类Order的对象无法转换为字符串

Question

实际上我在行号-34中遇到错误（指向函数check_id，if（codition））并且我不明白为什么会显示错误，这是什么意思和帮助SomeOne解释我以及我如何解决它（我是PHP新手）： 这是我的php文件：

    <?php
 include_once("Order.php");
 include("connect.php");
  $query="SELECT * FROM `orders`";
$filter_Result=mysqli_query($con,$query);
$newOrders=Array();  

while($row=mysqli_fetch_array($filter_Result))
 {
$order;
 $orderId= $row['id']; //fetch row id
 echo "hello".$orderId;

 if(check_id($newOrders,$orderId)){
  echo "kim";
  //$order= Add the order object
  }
  else{
echo " jim<br>";
 $order=new Order($row['id'], $row['tableId'], $row['createdDate']);
 array_push($newOrders,$order);
  }
 $item=new Item($row['ProductId'], $row['ProductName'], $row['Quantity']);
 //include("Order.php");
 $order->AddItem($item,null,null,null);

 }
 function check_id($newOrders,$orderId){
 $length=count($newOrders);
  echo $length;
 for($i=0; $i<$length; $i++){

if($newOrders[$i].$orderId == $orderId)
return true;
}
 return false;
 }

   ?>