Display posts order by custom post in a dropdown menu

Question

In my WordPress backend I have some custom posts and, in one of them, I want to display in a dropdown field some posts related with but ordered by post type. Iwant to display something like this.

<select multiple="multiple" style="height:200px; width:300px">
<optgroup label="Productos">
<option>Producto 1</option>
<option>Producto 2</option>
<option>Producto 3</option>
<option>Producto 4</option>
<option>Producto 5</option>
<option>Producto 6</option>
</optgroup>
<optgroup label="Paso 1">
<option>Subpagina 1</option>
<option>Subpagina 2</option>
<option>Subpagina 3</option>
</optgroup>
</optgroup>
<optgroup label="Paso 2">
<option>Subpagina 1</option>
<option>Subpagina 2</option>
<option>Subpagina 3</option>
</optgroup>
</optgroup>
<optgroup label="Paso 3">
<option>Subpagina 1</option>
<option>Subpagina 2</option>
<option>Subpagina 3</option>
</optgroup>
</select>

Here is the code that displays all of posts that I want but without order. I figure out that I have to use the id of the custom post type but I do not how to:

{
// Add the Meta Box  

function add_custom_meta_box_related() {  
    add_meta_box(  
        'custom_meta_box_related', // $id  
        'Related Information', // $title   
        'show_custom_meta_box_related', // $callback  
        'related', // $page  
        'normal', // $context  
        'high'); // $priority  
}  
add_action('add_meta_boxes', 'add_custom_meta_box_related');

    // Field Array  
    $prefix_related = 'custom_';  
    $custom_meta_fields_related = array(  

        array(  
            'label' => 'Related Items',  
            'desc' => 'Select a related item(s)',  
            'id'    =>  $prefix_materiales.'post_id',  
            'type' => 'post_list',  
            'post_type' => array('products','paso1','paso2','paso3','paso4','compra'),  
)
    );  

    // The Callback  
function show_custom_meta_box_related() {  
global $custom_meta_fields_related, $post;  
// Use nonce for verification  
echo '<input type="hidden" name="custom_meta_box_nonce" value="'.wp_create_nonce(basename(__FILE__)).'" />';  

    // Begin the field table and loop  
    echo '<table class="form-table">';  
    foreach ($custom_meta_fields_related as $field_related) {  
        // get value of this field if it exists for this post  
        $meta_related = get_post_meta($post->ID, $field_related['id'], true);  
        // begin a table row with  
        echo '<tr> 
                <th><label for="'.$field_related['id'].'">'.$field_related['label'].'</label></th> 
                <td>';  
                switch($field_related['type']) {  
                    // case items will go here 

                    // post_list  
                    case 'post_list':  
                    $items = get_posts( array (  
                        'post_type' => $field_related['post_type'],  
                        'posts_per_page' => -1  
                    ));  
                    echo '<select multiple name="'.$field_related['id'].'" id="'.$field_related['id'].'"> 
                    <option value="">Select One or more</option>'; // Select One  
                    foreach($items as $item) {  
                        echo '<option value="'.$item->ID.'"',$meta_related == $item->ID ? ' selected="selected"' : '','> '.$item->post_title.'</option>';  
                    } // end foreach  
                    echo '</select><br /><span class="description">'.$field_related['desc'].'</span>';  
                    break;
                } //end switch  
        echo '</td></tr>';  
    } // end foreach  
    echo '</table>'; // end table  
}

// Save the Data  
function save_custom_meta_related($post_id) {  
    global $custom_meta_fields_related;  

    // verify nonce  
    if (!wp_verify_nonce($_POST['custom_meta_box_nonce'], basename(__FILE__)))   
        return $post_id;  
    // check autosave  
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE)  
        return $post_id;  
    // check permissions  
    if ('page' == $_POST['post_type']) {  
        if (!current_user_can('edit_page', $post_id))  
            return $post_id;  
        } elseif (!current_user_can('edit_post', $post_id)) {  
            return $post_id;  
    }  

    // loop through fields and save the data  
    foreach ($custom_meta_fields_related as $field_related) {  
        $old = get_post_meta($post_id, $field_related['id'], true);  
        $new = $_POST[$field_related['id']];  
        if ($new && $new != $old) {  
            update_post_meta($post_id, $field_related['id'], $new);  
        } elseif ('' == $new && $old) {  
            delete_post_meta($post_id, $field_related['id'], $old);  
        }  
    } // end foreach  
}  
add_action('save_post', 'save_custom_meta_related');


}

Answer 1

Now I can order de post_types. Any help for the multiple save?

// The Callback  

function show_custom_meta_box_related() {
global $custom_meta_fields_related, $post;
// Use nonce for verification
echo '';

// Begin the field table and loop  
echo '<table class="form-table">';  
foreach ($custom_meta_fields_related as $field_related) {  
    // get value of this field if it exists for this post  
    $meta_related = get_post_meta($post->ID, $field_related['id'], true);  
    // begin a table row with  
    echo '<tr> 
            <th><label for="'.$field_related['id'].'">'.$field_related['label'].'</label></th> 
            <td>';  
            switch($field_related['type']) {  
                // case items will go here 

                // post_list  
                case 'post_list': 
                echo '<select multiple style="height:200px; width:300px" name="'.$field_related['id'].'" id="'.$field_related['id'].'">';
                echo '<option value=""></option>'; // Select One
                foreach($field_related['post_type'] as $tipo_post){
                $items = get_posts( array (  
                    'post_type' => $tipo_post,  
                    'posts_per_page' => -1  
                )); 

                foreach($items as $item) {  
                    echo '<option value="'.$item->ID.'"',$meta_related == $item->ID ? ' selected="selected"' : '','> '.$item->post_title. '</option>';  
                } // end foreach

                }
                echo '</select><br /><span class="description">'.$field_related['desc'].'</span>';  
                break;
            } //end switch  
    echo '</td></tr>';  
} // end foreach  
echo '</table>'; // end table  

}