[php] 2.3 - fix form params json encoding (#5701)

* fixed tests expectations not compatible with phpunit 4

* added test and endpoint definition, updated template

* regenerated sample

* regenerated security sample

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

@@ -0,0 +1,34 @@
+<?php
+
+namespace Swagger\Client;
+
+use Swagger\Client\Api\FakeApi;
+
+class RequestTest extends \PHPUnit_Framework_TestCase
+{
+
+    /** @var FakeApi */
+    private $api;
+    /** @var  FakeHttpClient */
+    private $fakeClient;
+
+    public function setUp()
+    {
+        $this->fakeClient = new FakeHttpClient();
+        $this->api = new Api\FakeApi($this->fakeClient);
+    }
+
+    public function testFormDataEncodingToJson()
+    {
+        $this->api->testJsonFormData('value', 'value2');
+
+        $request = $this->fakeClient->getLastRequest();
+        $contentType = $request->getHeader('Content-Type');
+        $this->assertEquals(['application/json'], $contentType);
+
+        $requestContent = $request->getBody()->getContents();
+
+        $expected = json_encode(['param' => 'value', 'param2' => 'value2']);
+        $this->assertEquals($expected, $requestContent);
+    }
+}