Error in PHP page

<?php
session_start();
class add
{
    var $conn;
   
    function __construct()
    {
        $this->conn=new MySQLi("localhost","root","","bis_tally");
    }
   
   
   
   
    //-----------------------------------
    function addStudent($user_name,$user_pass,$user_email,$user_contact)
    {
        $addUser=$this->conn->prepare("INSERT INTO                           `user`(user_name,user_pass,user_email,user_contact)VALUES(?,?,?,?);");
        $addUser->bind_param("ssss",$user_name,$user_pass,$user_email,$user_contact);
        $addUser->execute();
        //--------------error
    header("location:index.php");   
    echo "you are registred"    ;
    //-----------------
    }
   
    function addBuyer($company_address,
                        $buyer_name,
                        $invoice_number,
                        $date,$delivery_note,
                        $terms_of_payment,
                        $supplier_reference,
                        $other_reference,
                        $buyer_order_number,
                        $date_1,
                        $despatch_doc_number,
                        $date_2,
                        $despatched_through,
                        $destination,
                        $terms_of_delivery,   
                        $particular,
                        $quantity,
                        $rate,
                        $percentile,
                        $amount,
                        $service_tax)
    {
       
        $addInfo=$this->conn->prepare("INSERT INTO `buyer`(company_address,buyer_name,invoice_number,
        date,delivery_note,terms_of_payment,supplier_reference,other_reference,buyer_order_number,date_1,despatch_doc_number,date_2,
        despatched_through,destination,terms_of_delivery,particular,quantity,rate,percentile,amount,service_tax)VALUES(?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?);");
       
        $addInfo->bind_param("ssisssssssssssssiiiis",$company_address,$buyer_name,$invoice_number,
        $date,$delivery_note,$terms_of_payment,$supplier_reference,$other_reference,$buyer_order_number,
        $date_1,$despatch_doc_number,$date_2,$despatched_through,$destination,$terms_of_delivery,$particular,$quantity,
        $rate,$percentile,$amount,$service_tax);
        $addInfo->execute();
       
    }//-------------------------new function for amount  --------------------
        function addAmount($invoice_number,$quantity,$rate,$percentile,$amount)
        {
        $addValue=$this->conn->prepare("INSERT INTO `amount2`(invoice_number,quantity,rate,percentile,amount) VALUES (?,?,?,?,?);");
        $addValue->bind_param("iiiii",$invoice_number,$quantity,$rate,$percentile,$amount);
        $addValue->execute();
        }
        //--------------------------------------------------------------------
   

    function login($user_name,$user_pass)
      {
          $login=$this->conn->prepare("SELECT * FROM `user` WHERE `user_name`='".$user_name."' AND `user_pass`='".$user_pass."'");
          $login->bind_param("ss",$user_name,$user_pass);
          $login->execute();
          $login->store_result();
          if($login->num_rows == 0)
            {
                return false;
            }
            else
            {
                return true;
            }
      }
    // next function..... contd...
   
 }
?>


very important concept of SSSSS.....
contd....