some fix

common/models/UseField.php

@@ -39,7 +39,7 @@ class UseField extends \yii\db\ActiveRecord
         return [
             [['field_id', 'use'], 'required'],
             [['field_id', 'use'], 'integer'],
-            [['field_id'], 'exist', 'skipOnError' => true, 'targetClass' => AdditionalFields::className(), 'targetAttribute' => ['field_id' => 'id']],
+            [['field_id'], 'exist', 'skipOnError' => true, 'targetClass' => AdditionalFields::class, 'targetAttribute' => ['field_id' => 'id']],
         ];
     }
 

composer.json

@@ -14,7 +14,7 @@
     },
     "minimum-stability": "stable",
     "require": {
-        "php": ">=5.4.0",
+        "php": ">=7.1.0",
         "yiisoft/yii2": "~2.0.6",
         "yiisoft/yii2-bootstrap": "~2.0.0",
         "yiisoft/yii2-swiftmailer": "~2.0.0 || ~2.1.0",