Adds a "Transaction Number" column to the Event Espresso > Registrations page. Event Espresso 4

custom_column_registrations.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter(
    'FHEE_manage_event-espresso_page_espresso_registrations_columns',
    'my_filter_registration_list_table_columns',
    10,
    2
);
add_action(
    'AHEE__EE_Admin_List_Table__column_txnnumber__event-espresso_page_espresso_registrations',
    'my_registration_list_table_txnnumber_column', 
    10, 
    2
);

/**
 * this function adds the column name to the array of table headers
 *
 * @param array $columns
 * @param string $screen
 * @return array
 */
function my_filter_registration_list_table_columns( $columns, $screen ) {
    if ( $screen === "espresso_registrations_default" ) {
        $offset = isset($columns['TXN_paid']) ? 'TXN_paid' : '_REG_paid';
        $columns = EEH_Array::insert_into_array(
            $columns,
            array( 'txnnumber' => 'Transaction Number' ),
            $offset,
            false
        );
    }
    return $columns;
}

/**
 * this function echoes out the data you want to appear in your custom column.
 *
 * @param \EE_Registration $item
 * @param string           $screen
 */
function my_registration_list_table_txnnumber_column( $item, $screen ) {
    if ( $screen === "espresso_registrations_default" && $item instanceof EE_Registration ) {
        $txn = $item->transaction();
        if ($txn instanceof EE_Transaction) {
            $txn_payments = $txn->approved_payments();
            foreach($txn_payments as $txn_payment) {
                if($txn_payment instanceof EE_Payment) {
                    echo $txn_payment->txn_id_chq_nmbr() . '<br>';
                }
            }
        }
    }
}