How to store the value of a custom field dropdown select for post referencing?

Question

I'm trying to reference posts to each other. Editors should have a custom field select list of certain post's titles to link the current post with other posts. I got quite far, I think. The select list appears correctly. All titles are there.

Now, I'm stuck at the point, where the referred post's ID has to be saved.

How do I grab the selected option's value, save it and make it default value? Do I have to wrap everything inside a form function?

This is what I have so far:

add_action("admin_init", "admin_init");
add_action('save_post', 'save_reference_id');  
/*** add_meta_box ***/
function admin_init(){
    // adding a custom field to post type 'case'
    add_meta_box("refInfo-meta", "Reference", "meta_options", "case", "side", "high");
}  
/*** callback ***/
function meta_options(){
  ?>
    <form action="<?php bloginfo('url'); ?>" method="get">
      <select name="page_id" id="page_id">

      <?php
      global $post;
      // getting all child pages of ID 21
      $args = array( 'numberposts' => -1, 'post_type' => 'page', 'post_parent' => 21);
      $posts = get_posts($args);

      foreach( $posts as $post ) : setup_postdata($post); ?>
        <option value="<?php echo $post->ID; ?>"><?php the_title(); ?></option>
      <?php endforeach; ?>

      </select>
    </form>

  <?php
}  
/*** save_post ***/
function save_reference_id(){
  // big question mark
}

Answer 1

selected() was big help for setting a default value. The rest I found in this brilliant meta box tutorial: http://code.tutsplus.com/tutorials/how-to-create-custom-wordpress-writemeta-boxes--wp-20336 with examples for text input, checkboxes and dropdown. Also Custom post type's slug gets wrong when adding a custom meta box explained me how to correctly handle the current post object so it doesn't get mixed up with the options objects.

/*** callback ***/
function meta_options(){
  global $post;
  // storing the global post object so it doesn't get mixed up with the options
  $post_old = $post

  $custom = get_post_custom($post->ID);
  if (isset($custom["reference_id"][0])) {
    $reference_id = $custom["reference_id"][0];
  } else {
    $reference_id = '0';
  }
  ?>
    <form action="<?php bloginfo('url'); ?>" method="get">
      <select name="ref_id" id="ref_id">
        <option value="0" <?php selected($reference_id, '0'); ?>>- choose client -</option>
      <?php
      global $post;
      $args = array(
        'numberposts' => -1,
        'post_type' => 'page',
        'post_parent' => 21
        );
      $posts = get_posts($args);
      foreach( $posts as $post ) : setup_postdata($post); ?>
        <option value="<?php echo $post->ID; ?>" <?php selected($reference_id, $post->ID); ?>><?php the_title(); ?></option>
      <?php endforeach; ?>
      </select>
    </form>
  <?php
  // restoring the global post object
  $post = $post_old;
  setup_postdata( $post );
}

/*** save_post ***/
function save_reference_id(){
  global $post;
  if (isset($_POST["ref_id"])) {
    update_post_meta($post->ID, "reference_id", $_POST["ref_id"]);
  }
}

Answer 2

add_action("admin_init", "admin_init");
add_action('save_post', 'save_reference_id');  
/*** add_meta_box ***/
function admin_init(){
    // adding a custom field to post type 'case'
    add_meta_box("refInfo-meta", "Reference", "meta_options", "case", "side", "high");
}  
/*** callback ***/
function meta_options(){
  ?>
  <select name="page_id" id="page_id">
<?php
  global $post;
  // getting all child pages of ID 21
  $args = array( 'numberposts' => -1, 'post_type' => 'page', 'post_parent' => 21);
  $posts = get_posts($args);

  foreach( $posts as $post ) : setup_postdata($post); ?>
    <option value="<?php echo $post->ID; ?>"><?php the_title(); ?></option>
  <?php endforeach; ?>
  </select>    
  <?php
}  
/*** save_post ***/
function save_reference_id($post_id){
    update_post_meta($post_id, "[meta_key]", $_POST['page_id']);
}