Store entrance code in meta.

src/Gateway.php

@@ -141,14 +141,32 @@ public function start( Payment $payment ) {
 
 		$payment->set_meta( 'purchase_id', $purchase_id );
 
+		/**
+		 * The Transaction.entranceCode is an 'authentication identifier' to
+		 * facilitate continuation of the session between Merchant and Consumer,
+		 * even if the existing session has been lost. It enables the Merchant to
+		 * recognise the Consumer associated with a (completed) transaction.
+		 * The Transaction.entranceCode is sent to the Merchant in the Redirect.
+		 * The Transaction.entranceCode must have a minimum variation of 1
+		 * million and should comprise letters and/or figures (maximum 40
+		 * positions).
+		 * The Transaction.entranceCode is created by the Merchant and passed
+		 * to the Issuer.
+		 *
+		 * @link https://www.pronamic.eu/wp-content/uploads/sites/2/2016/06/Merchant-Integration-Guide-v3-3-1-ENG-February-2015.pdf
+		 */
+		$entrance_code = \wp_generate_password( 40 );
+
+		$payment->set_meta( 'entrance_code', $entrance_code );
+
 		// Transaction.
 		$transaction = new Transaction();
 		$transaction->set_purchase_id( $purchase_id );
 		$transaction->set_amount( $payment->get_total_amount()->number_format( null, '.', '' ) );
 		$transaction->set_currency( $payment->get_total_amount()->get_currency()->get_alphabetic_code() );
 		$transaction->set_expiration_period( 'PT30M' );
 		$transaction->set_description( $payment->get_description() );
-		$transaction->set_entrance_code( $payment->get_entrance_code() );
+		$transaction->set_entrance_code( $entrance_code );
 
 		$customer = $payment->get_customer();