[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

samples/client/petstore/php/SwaggerClient-php/lib/Configuration.php

@@ -52,7 +52,6 @@ namespace Swagger\Client;
  */
 class Configuration
 {
-
     private static $defaultConfiguration = null;
 
     /**
@@ -60,14 +59,14 @@ class Configuration
      *
      * @var string[]
      */
-    protected $apiKeys = array();
+    protected $apiKeys = [];
 
     /**
      * Associate array to store API prefix (e.g. Bearer)
      *
      * @var string[]
      */
-    protected $apiKeyPrefixes = array();
+    protected $apiKeyPrefixes = [];
 
     /**
      * Access token for OAuth
@@ -95,7 +94,7 @@ class Configuration
      *
      * @var array
      */
-    protected $defaultHeaders = array();
+    protected $defaultHeaders = [];
 
     /**
      * The host
@@ -493,7 +492,7 @@ class Configuration
      */
     public static function getDefaultConfiguration()
     {
-        if (self::$defaultConfiguration == null) {
+        if (self::$defaultConfiguration === null) {
             self::$defaultConfiguration = new Configuration();
         }