Bookings requiring another event booking

<?php
/**
 * The function below requires a previous event to be booked, or for that required event to be present in the booking cart.
 * Once installed, add a new custom field called 'Parent Event ID' and then add the event ID of the required event booking to your secondary event.
 * For instructions on how to install this snippet, see here - http://wp-events-plugin.com/tutorials/how-to-safely-add-php-code-to-wordpress/
*/

/**
 * @param boolean $result
 * @param EM_Booking $EM_Booking
 */
function my_em_booking_validate_dependence( $result, $EM_Booking ){
    //check if the event has a designated parent event
    $EM_Event = $EM_Booking->get_event();
    if( !empty($EM_Event->event_attributes['Parent Event ID']) ){
        //get the parent event and make sure it exists
        $EM_Event = em_get_event($EM_Event->event_attributes['Parent Event ID']);
        if( empty($EM_Event->event_id) ) return $result;
        $has_booked_parent_event = false;
        //if user isn't logged in, we fail validation as they must log in so we can verify their booking of the parent event. If MB is enabled, we skip since they may have booked both events at once
        if( !is_user_logged_in() && !get_option('dbem_multiple_bookings') ){
            $EM_Booking->add_error("You must have previously booked '{$EM_Event->event_name}' before this event. Please log in so we can verify your previous bookings.");
            return false;
        }
        //check if user has previously boooked this parent event
        if( is_user_logged_in() ){
            //make sure we search all events 'owner'=>false and only booked or pending offline payment events 'status'=>'1,5'
            $EM_Bookings = EM_Bookings::get( array('person' => get_current_user_id(), 'event' => $EM_Event->event_id, 'owner'=>false, 'status' => '1,5') );
            if( !empty($EM_Bookings->bookings) ){
                $has_booked_parent_event = true;
            }
        }
        //check if this is booking mod and the event is already in the cart
        if( !$has_booked_parent_event && get_option('dbem_multiple_bookings') ){
            $EM_Multiple_Booking = EM_Multiple_Bookings::get_multiple_booking();
            foreach( $EM_Multiple_Booking->get_bookings() as $em_booking ){ /* @var EM_Booking $em_booking */
                if( $em_booking->event_id == $EM_Event->event_id ){
                    $has_booked_parent_event = true;
                    break;
                }
            }
        }
        //if parent booking not found, provide error message and fail validation
        if( !$has_booked_parent_event ){
            $EM_Booking->add_error("You must book '".$EM_Event->output('#_EVENTLINK')."' in order to attend this event.");
            $result = false;
        }
    }
    return $result;
}
add_filter('em_booking_validate', 'my_em_booking_validate_dependence', 10, 2);

function my_em_multiple_booking_validate_dependence( $result, $EM_Multiple_Booking ){
    foreach( $EM_Multiple_Booking->get_bookings() as $EM_Booking ){ /* @var EM_Booking $EM_Booking */
        if( !my_em_booking_validate_dependence( true, $EM_Booking ) ){
            $result = false;
        }
    }
    return $result;
}
add_filter('em_multiple_booking_validate','my_em_multiple_booking_validate_dependence', 10, 2);