WordPress custom metabox implementation

parent_metabox.php

<?php

/** 
 * parent metabox implementation
 */

 // hook
 add_action( 'add_meta_boxes', 'CustomMetabox_setup' );
 
 /**
 * callback from hook
 */
function CustomMetabox_setup() {

    add_meta_box(
            'customer_parent', 		//Id Metabox
            'MetaBox Title', 		//Title MetaBox
            'customer_parent_metabox', //Callback Output function
            'page', 			//(post,page,custom post type)
            'side',			//admin area
            'high'			//priority
            );
		
}

/**
* metabox render function 
*/
function customer_parent_metabox(){
		
        global $post;

        $out = "";

        $args = array(
                'post_type' => 'forum', 
                'post_status' => 'publish',
                'orderby' => 'title',
                'order' => 'ASC',			
                );


        $customer = get_transient( 'customer_parent_metabox' );
        if ( false == $customer ) {
                $customer = get_posts( $args );
                set_transient( 'customer_parent_metabox', $customer );
        } 


        $out .= "<p><i>Seleziona l'azienda da associare</i></p>";
        $out .= wp_combo_posts("parent_id", $customer , $post->post_parent , "full_width");


        $out .= "<p><a href='post.php?post=" . $post->post_parent . "&action=edit'>vai alla scheda di dettaglio</a></p>";


    	echo $out;
		
		}
		

/**
* utils -  generate select 
*/		
function wp_combo_posts($id, $data, $selected = '' , $class='') {

    $options = "";
	$options .= "<option value=''></option>";
	
	foreach ($data as $post) {
            
		
		$selected_attribute = ( $post->ID == $selected ? " selected " : "" );

		// applico il filtro per gestione multilingua
		$title = apply_filters('the_title', $post->post_title);
		$options .= "<option " . $selected_attribute . " value='" . $post->ID . "'>" .  $title  . "</option>";

}

    
	 
	return "<select id='$id' name='$id' class='$class' >$options</select>";
	}