Add PHP example using generated client code

Signed-off-by: Oliver Davies <oliver@oliverdavies.uk>

openapi/example.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+require_once __DIR__.'/out/php/vendor/autoload.php';
+
+use GuzzleHttp\Client;
+use OpenAPI\Client\Api\PetApi;
+use OpenAPI\Client\Configuration;
+
+$config = Configuration::getDefaultConfiguration()
+    ->setApiKey('api_key', 'YOUR_API_KEY');
+
+$api = new PetApi(new Client(), $config);
+
+$pet = $api->getPetById(56);
+
+var_dump($pet);