Stick to definition.

Author: andris-sevcenko

src/base/CreditCardGateway.php

@@ -36,7 +36,7 @@ public function getPaymentFormHtml(array $params)
     /**
      * @inheritdoc
      */
-    public function getPaymentFormModel()
+    public function getPaymentFormModel(): BasePaymentForm
     {
         return new CreditCardPaymentForm();
     }

src/base/Gateway.php

@@ -33,6 +33,10 @@
 use yii\base\NotSupportedException;
 
 /**
+ * Class Gateway
+ *
+ * @property string $itemBagClassName
+ * @property string $gatewayClassName
  * @author    Pixel & Tonic, Inc. <[email protected]>
  * @since     1.0
  */

src/base/OffsiteGateway.php

@@ -14,7 +14,7 @@ abstract class OffsiteGateway extends Gateway
     /**
      * @inheritdoc
      */
-    public function getPaymentFormModel()
+    public function getPaymentFormModel(): BasePaymentForm
     {
         return new OffsitePaymentForm();
     }