Passing variables html forms in PHP securely

I have a form that is populated by data from a MySQL database. To make an update/edit to the form I would like to use the primary key of that record. How can I pass the primary key to the page securely once the page is submitted. I do not want to use $_GET as any user can change this on the URL and embedding the primary key in a hidden form field can also be sabotaged and is visible in the html source. The action is being done on the same page(see code block below). The processing is being done on the same page so I am not sure whether sessions will work.

//Load the data from the database
  if(isset($_GET['menu_id'])){
       $menu_id = (int)$_GET['menu_id'];
       $menu = new Menu();
       $menu_item = $menu->get_menu_items_by_id($menu_id);
  }
  else{
       //The user has possibly edited the URL
       $message = "Please select an option to edit";
       redirect_to($PHP_SELF . '?message=' . $message);
  }
?>
<form method="POST" action="<?php echo $_SERVER['PHP_SELF'] ?>">

<div>
<label>Name</label>
<input type="text" name="name" value="<?php echo $menu_item->name ?>">
</div>


<div>
<label>Title</label>
<input type="text" name="title" value="<?php echo $menu_item->title ?>">
</div>


<div>
<label>Is default page</label>
<input type="radio" name="is_default_page" value="1" 
    <?php 
        if($menu_item->is_default_page == 1) 
            {
            echo "checked"; 
            }?>
     >Yes
<input type ="radio" name="is_default_page" value="0" 
        <?php 
        if($menu_item->is_default_page == 0) 
            {
            echo "checked"; 
            }?>

     >No    
</div>

<div>
<label>Page name</label>
<input type="text" name="page" value="<?php echo $menu_item->page ?>" />
</div>

<div>
<label>Menu type</label>
<select name="menu_type">
    <?php
    //Display the options of the menu types available
    $menu_type_array = $menu->get_menu_types();
    draw_select($menu_type_array, 'name', 'id', $menu->menu_type_id);
    ?>
</select>
</div>

<div>
    <label>Page type i.e what is the page used for</label>
    <select name="page_type">
        <?php
        $page = new Page();
        $page_type_array  = $page->get_page_types();
        draw_select($page_type_array, 'name', 'id', $menu->page_type_id)
        ?>
    </select>
</div>

<div>
    <label>Position</label>
    <select name="position">
        <?php
        //Count the number of menus in the database

        $number_of_menu_items = $menu->count_menu_items() + 1;
        for($i=1; $i<=$number_of_menu_items; $i++){
            echo "<option value=\"{$i}\"";
            if($i==$menu->position){
                echo "selected = \"selected\"";
            }
            echo ">";
            echo "{$i}";
            echo "</option>";
        }

        ?>
    </select>
</div>

<div>
    <input type="submit" value="Add Menu" name="edit_menu" />
</div>


<form>