Custom Post type meta box for WordPress (extension to CPT)

Custom Post type meta box for WordPress

/******* RELATED PAGES METABOX *********/
add_action( 'cmb_render_select_post', 'rrh_cmb_render_select_post', 10, 2 );
function rrh_cmb_render_select_post( $field, $meta ) {
    global $posts;
    $posts = get_posts( array( 'post_type' => 'page', 'posts_per_page' => -1 ));
    echo '<select name="', $field['id'], '" id="', $field['id'], '">';
         foreach ($posts as $option) {
        echo '<option value="', $option->ID, '"', $meta == $option->ID ? ' selected="selected"' : '', '>', $option->post_title,     '</option>';
        }
        echo '</select>';
        echo '<p class="cmb_metabox_description">', $field['desc'], '</p>';
 }	
	
$meta_boxes[] = array(
		'id' => 'related_pages',
		'title' => 'Related Pages',
		'pages' => array('page'), // post type
		'context' => 'normal',
		'priority' => 'high',
		'show_names' => true, // Show field names on the left
		'fields' => array(
			array(
				'name' => 'Previous Page',
    			'desc' => '',
   				'id' => $prefix . 'prev_page',
    			'type' => 'select_post',
			),
			
			array(
				'name' => 'Next Page',
    			'desc' => '',
   				'id' => $prefix . 'next_page',
    			'type' => 'select_post',
			),
			
			
		
			
		)
	);