Adds a column for every datetime of the event and displays an "x" if there's a checkin for that datetime. A variation of https://github.com/eventespresso/ee-code-snippet-library/blob/master/admin/jf_ee_registration_report_add_checkin_timestamps.php

checkins_to_csv.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file
add_filter( 'FHEE__EventEspressoBatchRequest__JobHandlers__RegistrationsReport__reg_csv_array', 
    'espresso_add_checkin_timestamp_csv_report', 
    10, 
    2
);
function espresso_add_checkin_timestamp_csv_report( array $csv_row, $reg_db_row ) {
    
    $checkin_rows = (array)EEM_Checkin::instance()->get_all_wpdb_results(
        array(
            array(
                'REG_ID' => $reg_db_row['Registration.REG_ID'],
            ),
        )
    );
    $checkins_for_csv_col = array();
    $datetime_checkins_for_csv_col = array();
    $checkin_dtt_id_array = array();
    foreach ( $checkin_rows as $checkin_row ) {
        $checkins_for_csv_col[] = $checkin_row['Checkin.CHK_timestamp'];
        $checkin_for_dtt_id = $checkin_row['Checkin.DTT_ID'];
        $checkin_for_dtt_name = \EEM_Datetime::instance()->get_var(
          array(
            array('DTT_ID' => $checkin_for_dtt_id)
          ),
        'DTT_name');
        $datetime_checkins_for_csv_col[] = $checkin_for_dtt_name ?
         $checkin_for_dtt_name . ' - ID ' . $checkin_for_dtt_id :
         $checkin_for_dtt_id;
        $checkin_dtt_id_array[] = $checkin_for_dtt_id;
    }
    foreach (
        \EEM_Datetime::instance()->get_all_wpdb_results(array(
            array('Ticket.TKT_ID' => $reg_db_row['Ticket.TKT_ID']),
            'order_by'                 => array('DTT_EVT_start' => 'ASC'),
            'default_where_conditions' => 'none',
        )) as $datetime
    ) { 
      $checked_in = '';
       if (in_array($datetime['Datetime.DTT_ID'], $checkin_dtt_id_array ) ) {
          $checked_in = 'x'; 
        }
        $csv_row[ (string) \EEH_Export::prepare_value_from_db_for_display(\EEM_Datetime::instance(),
            'DTT_EVT_start', $datetime['Datetime.DTT_EVT_start']) ] = $checked_in;
    }
    $csv_row[ (string)__( 'Checked in for Datetime', 'event_espresso' ) ] = implode( ' + ', $datetime_checkins_for_csv_col );
    $csv_row[ (string)__( 'Checkin timestamps', 'event_espresso' ) ] = implode( ' + ', $checkins_for_csv_col );
    return $csv_row;
}