add_action(
  'AHEE__EE_Registration_Processor__trigger_registration_update_notifications',
  'act_ee4_assign_invoicenr', 10, 2
); // 1, 2

function act_ee4_assign_invoicenr( $registration, $update_params ) {

  // get registration status. $registration is passed through the hook
  $registration_status = $registration->status_ID();

  $reg_status_value = EEM_Status::instance()->get_var(
     array(
        array(
          'Registration.STS_ID' => $registration_status
        ),
        'limit' => 1
      ),
      'STS_code'
  );

  // get transaction
  $transaction = $registration->transaction();
  $transaction_status = $transaction->status_ID();

  // admin question id
  $invoice_question_id = 21;


  // Assemble custom invoice number if the registration is approved

  if ($registration_status == "RAP") {

    // get invoice number, year and prefix from options table
    $current_year = (int) date( 'Y' );
    $invoice_prefix = "20"; //get_option( 'invoice_prefix' );
    $invoice_year = (int) get_option( 'invoice_year' ); // e.g. 2017
    $invoice_order = (int) get_option( 'invoice_order' ); // e.g. 24

    // increment year value if current year is higher
    if ( $invoice_year != $current_year ) {
      update_option( 'invoice_year', $current_year );
      update_option( 'invoice_order', 1 );
    }

    // increment invoice number for next invoice
    update_option( 'invoice_order', $invoice_order + 1 );

    // assemble invoice number
    $invoice_number = $invoice_prefix . $invoice_year
      . str_pad( $invoice_order, 4, '0', STR_PAD_LEFT );

  }
  else{
    // Otherwise temporarily populate the answer with the Registration Status Value
    $invoice_number = $reg_status_value;
  }



  // Assign Invoice Number

  // Check if there is an answer for this question already

  $prev_answer_value = EEM_Answer::instance()->get_answer_value_to_question($registration, $invoice_question_id, false);

  if( $prev_answer_value ) {

    // If answer exists, update it
    $prev_answer_object = EEM_Answer::instance()->get_registration_question_answer_object($registration, $invoice_question_id, false);
    $prev_answer_object->set('ANS_value', $invoice_number);
    $prev_answer_object->save();
  }
  else{

    // if it doesn't exist insert it
    EEM_Answer::instance()->insert(
      array(
        'REG_ID' => $registration->ID(),
        'QST_ID' => $invoice_question_id, // admin only question 'invoice number'
        'ANS_value' => $invoice_number
      )
    );
  }

}