refactor of Event Espresso 3's event_espresso_send_invoice() function so that each attendee gets a their own unique email reminder. Assumes usage of Multi Event Registration.

custom_functions.php

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

if (!function_exists('event_espresso_send_invoice')) {

	function event_espresso_send_invoice( $registration_id, $invoice_subject, $invoice_message, $array_of_reg_ids = FALSE ) {
		global $wpdb, $org_options;
		do_action('action_hook_espresso_log', __FILE__, __FUNCTION__, '');

		$SQL ='SELECT a.attendee_session FROM '. EVENTS_ATTENDEE_TABLE . ' a WHERE a.registration_id = %s';
		$result = $wpdb->get_var( $wpdb->prepare( $SQL, $registration_id ));
		
		$sql2 = 'SELECT a.id FROM ' . EVENTS_ATTENDEE_TABLE . ' a WHERE a.attendee_session = %s';
		$attendee_ids = $wpdb->get_col( $wpdb->prepare($sql2, $result ));
		
		foreach ( $attendee_ids as $attendee_id ) {
			email_by_attendee_id($attendee_id, $send_attendee_email = TRUE, $send_admin_email = false, $multi_reg = FALSE, $custom_data = array('email_type' => 'invoice', 'invoice_subject' => $invoice_subject, 'invoice_message' => $invoice_message));
		}
		return;
	}

}//End event_espresso_send_invoice()