Use single quotes in yaml files

symfony/framework-bundle/3.3/etc/packages/framework.yaml

@@ -1,5 +1,5 @@
 framework:
-    secret: "%env(APP_SECRET)%"
+    secret: '%env(APP_SECRET)%'
     #default_locale: en
     #csrf_protection: null
     #http_method_override: true
@@ -7,7 +7,7 @@ framework:
     # https://symfony.com/doc/current/reference/configuration/framework.html#handler-id
     #session:
     #    handler_id: session.handler.native_file
-    #    save_path: "%kernel.project_dir%/var/sessions/%kernel.environment%"
+    #    save_path: '%kernel.project_dir%/var/sessions/%kernel.environment%'
     #esi: ~
     #fragments: ~
     php_errors:

symfony/framework-bundle/3.3/etc/routing.yaml

@@ -1,6 +1,6 @@
 #index:
 #    path: /
-#    defaults: { _controller: "App\\Controller\\DefaultController::index" }
+#    defaults: { _controller: 'App\Controller\DefaultController::index' }
 
 # Depends on sensio/framework-extra-bundle:^3.0 and doctrine/annotations
 #controllers: