function jjm_add_custom_box(){
                add_meta_box(
                    'jjm_concert_box_id',           // Unique ID
                    'concert toevoegen',  // Box title
                    'jjm_custom_box_concert_html',  // Content callback, must be of type callable
                    'concert'                 // Post type
                );
                add_meta_box(
                    'jjm_artiest_box_id',           // Unique ID
                    'Voeg een nieuwe artiest toe',  // Box title
                    'jjm_custom_box_artiest_html',  // Content callback, must be of type callable
                    'artiest'               // Post type
                );
                add_meta_box(
                    'jjm_zaal_box_id',           // Unique ID
                    'zaal toevoegen',  // Box title
                    'jjm_custom_box_zaal_html',  // Content callback, must be of type callable
                    'zaal'                 // Post type
                );
                add_meta_box(
                    'jjm_concert_zaal_box_id',   // Unique ID
                    'Koppel zaal',        // Box title
                    'jjm_custom_box_concert_zaal_html',   // Content callback, must be of type callable
                    'concerts'              // Post type
                );
            }
            function jjm_custom_box_concert_zaal_html($post){
                $value_gekoppelde_zaal_id = get_post_meta($post->ID,'_concert_zaal', true);
                print("<h1>Koppel concert met zaal</h1>");

                $zalen = get_all_of_post_type('zaal');

                foreach($zalen as $zaal){
                    print("<input type='radio' name='concert_zaal' value='" . $zaal->ID . "' " .
                            ($zaal->ID == $value_gekoppelde_zaal_id ? "checked": "") . ">". $zaal->post_title ."</input>");
                }
            }
            function get_all_of_post_type( $type_name = '') {
                $items = array();
                if ( !empty( $type_name ) ) {
                    $args = array(
                        'post_type' => "{$type_name}",
                        'posts_per_page' => -1,
                        'order' => 'ASC',
                        'orderby' => 'title'
                    );
                    $results = get_posts( $args );
                }
                return $results;
            }