[PHP] Fix date format serialization (#5754)

* [PHP] Honor Swagger/OpenAPI 'date' format

Per spec
(https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#data-types),
DateTime instances defined as 'date' datatype need to be serialized as
defined by full-date - RFC3339, which has the format:

full-date       = date-fullyear "-" date-month "-" date-mday

ref:
https://xml2rfc.tools.ietf.org/public/rfc/html/rfc3339.html#anchor14

see #5531
fixes #5607

* [PHP] Add `date` and `date-time` serializer tests

See #5531
See #5607

* [PHP] Improve codestyle of generated code

* [PHP] Regenerate PHP Petstore sample

* [PHP] Regenerate PHP Security sample

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

@@ -29,6 +29,5 @@ class UserApiTest extends \PHPUnit_Framework_TestCase
             $response,
             "response string starts with 'logged in user session'"
         );
-
     }
 }