[PHP] Better PSR2 compatibility (#3863)

* feature(php-cs-fixer) add php-cs-fixer support

* feature(php-cs-fixer) tweak Mustache templates to fit PSR2

* feature(php-cs-fixer) bin/php-petstore.sh output

modules/swagger-codegen/src/main/resources/php/.php_cs

@@ -0,0 +1,18 @@
+<?php
+
+return Symfony\CS\Config::create()
+    ->level(Symfony\CS\FixerInterface::PSR2_LEVEL)
+    ->setUsingCache(true)
+    ->fixers(
+        [
+            'ordered_use',
+            'phpdoc_order',
+            'short_array_syntax',
+            'strict',
+            'strict_param'
+        ]
+    )
+    ->finder(
+        Symfony\CS\Finder\DefaultFinder::create()
+            ->in(__DIR__)
+    );