Add Kernel.stubphp (#220)

src/Stubs/common/Component/HttpKernel/Kernel.stubphp

@@ -0,0 +1,11 @@
+<?php
+
+namespace Symfony\Component\HttpKernel;
+
+class Kernel
+{
+    /**
+     * @var string
+     */
+    protected $environment;
+}

tests/acceptance/acceptance/Kernel.feature

@@ -0,0 +1,27 @@
+@symfony-5
+Feature: Kernel
+
+  Background:
+    Given I have Symfony plugin enabled
+
+  Scenario: MixedOperand error about $environment is not raised
+    Given I have the following code
+      """
+      <?php
+
+      use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
+      use Symfony\Component\HttpKernel\Kernel as BaseKernel;
+      use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
+
+      abstract class Kernel extends BaseKernel
+      {
+          use MicroKernelTrait;
+
+          protected function configureContainer(ContainerConfigurator $container): void
+          {
+              $container->import('../config/{packages}/' . $this->environment . '/*.yaml');
+          }
+      }
+      """
+    When I run Psalm
+    Then I see no errors