Test phpstan stuff

Author: marius

src/Type/Definition/EnumType.php

@@ -132,10 +132,13 @@ public function getValues(): array
     public function serialize($value)
     {
         $lookup = $this->getValueLookup();
-        if ($value instanceof \BackedEnum) {
-            $value = $value->value;
-        } elseif ($value instanceof \UnitEnum) {
-            $value = $value->name;
+
+        if (class_exists(\BackedEnum::class)) {
+            if ($value instanceof \BackedEnum) {
+                $value = $value->value;
+            } elseif ($value instanceof \UnitEnum) {
+                $value = $value->name;
+            }
         }
         if (isset($lookup[$value])) {
             return $lookup[$value]->name;