add back php unit testing and readme to PHP SwaggerClient sample code

samples/client/petstore/php/SwaggerClient-php/tests/PetApiTest.php

@@ -0,0 +1,16 @@
+<?php
+require_once('SwaggerPetstore.php');
+class PetApiTest extends \PHPUnit_Framework_TestCase
+{
+  public function testGetPetById()
+  {
+    // initialize the API client
+    $api_client = new SwaggerPetstore\APIClient('http://petstore.swagger.io/v2');
+    $petId = 10005;  // ID of pet that needs to be fetched
+    $pet_api = new SwaggerPetstore\PetAPI($api_client);
+    // return Pet (model)
+    $response = $pet_api->getPetById($petId);
+    $this->assertSame($response->id, $petId);
+  }
+}
+?>