PHP object does not save correctly

[audiodef]

I have this OO PHP project that serializes an object and forwards it to the next page via the query string. The basic operation is fine, but instead of adding a new array element, it replaces the existing array element. Here's the code (with the non-relevant stuff cut out):

The class:

class User extends SystemUtils{

  var $default_page="index.php";
  var $order;

  function User(){
    /*
    Initializes order and sets up class vars.
    */

$orderdata=false;

extract($_GET,EXTR_IF_EXISTS);

$orderdata=unserialize($orderdata);

if($orderdata){
  foreach($orderdata as $k=>$v){
    $this->order[$k]=$v;
  }
}else{
  $this->order["order_date"]=date("Y-m-d");
}
  }

  function addPhotoToOrder($image_id){
    /*
    Adds quantity of 1 of specified image to order and returns user to gallery with serialized order info.
    Serialized order info is picked up by the class constructor when the page loads.
    */

$this->order[$image_id]=1; 
   //ABOVE DOESN'T ADD - JUST REPLACES, EVEN THOUGH $image_id IS DIFFERENT FROM THE EXISTING INDEX
    header("location:".$this->default_page."?orderdata=".serialize($this->order));
  }

}

index.php:

require_once("init.php");

$option="";
$password_req="";
$email_req="";
$image_id=0;
$user_id=false;

extract($_GET,EXTR_IF_EXISTS);
extract($_POST,EXTR_IF_EXISTS);
extract($_SESSION,EXTR_IF_EXISTS);

include($user->docroot."/html/head.htm");

if($user_id){
  //Logged in, proceed
  switch($option){
    case 'add_photo':
      $user->addPhotoToOrder($image_id);
    break;
    default:
      foreach($user->order as $k=>$v){
        echo "$k=>$v<br/>";
      }
      $gallery_name=$user->getGalleryName($user_id);
      $content=$user->getContent($user->docroot."/html/user/view_gallery.htm");
      $content=$user->viewGallery($gallery_name,$content);
      print $content;
  }
}else{
  //Not logged in, perform login function calls
  if($option=='login'){
    if($_POST){
      $credentials=serialize(array("password"=>$password_req));
      if($login_id=$user->authenticate($credentials,"user")){
        if($user->emailIsValid($email_req)){
          $email_id=$user->collectEmail($link,$email_req,$login_id);
          $user->login($email_id);
        }else{
          include($user->docroot."/html/messages/invalid_email.htm");
          include($user->docroot."/html/user/login.htm");
        }
      }else{
        include($user->docroot."/html/messages/login_failed.htm");
        include($user->docroot."/html/user/login.htm");
      }
    }else{
      include($user->docroot."/html/user/login.htm");
    }
  }else{
    include($user->docroot."/html/user/home.htm");
  }
}


include($user->docroot."/html/foot.htm");

What's supposed to happen is when $image_id is passed, it's supposed to add on to the $this->order array, but all it does is replace the existing, single index in that array. $image_id is different from the existing index, so I don't know how it's getting replaced instead of a new array element being created.

[Jasp182]

I don't totally understand what the code is supposed to do, but just judging by the names of the methods and variable names, don't you mean to add 1 to the array element (incrementing the number of photos in the order) as opposed to just setting it to 1?

If this isn't the case, have you tried debugging with echo statements to make absolutely sure that $image_id is set to what you expect it to be set to when passed to the addPhotoToOrder method?