shortcodes, custom php and their errors

The answer is:

function reservations_yesterday() {
global $wpdb;


$results = $wpdb->get_var ("SELECT SUM(`mattiscool`), booking_date FROM `wp_cbxrbooking_log_manager` WHERE `booking_date` = CURDATE() -1");

return $results;
}



add_shortcode('name', 'reservations_yesterday');