[php-nextgen] add php-nextgen client generator (#16480)

* php-nextgen first commit

* [php] Set minimal PHP version to ^8.0 (#14500)

* Set minimal PHP version to ^8.0

* Fix php-nextgen config

* Change stability to BETA

* Add phplint package (#15054)

* [php-nextgen] Rename folders to follow PDS skeleton (#15102)

* Change lib -> src, test -> tests folder

This will make build compliant to PHP-PDS skeleton.
Ref: https://github.com/php-pds/skeleton

* Refresh samples

* Exclude composer.lock from codebase (#15105)

Since client generator is library and not a project it makes sense
to exclude composer.lock from codebase by default.

Ref: http://getcomposer.org/doc/02-libraries.md#lock-file

* Add @generated tag to DocBlocks (#15106)

This tag in draft status right now(PSR-19), but I think we can leverage
from it already.

Ref: https://github.com/php-fig/fig-standards/blob/master/proposed/phpdoc-tags.md#55-generated

* update samples, doc

* update samples

---------

Co-authored-by: Yuriy Belenko <yura-bely@mail.ru>

samples/client/petstore/php-nextgen/OpenAPIClient-php/test/UserApiTest.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace OpenAPI\Client;
+
+use OpenAPI\Client\Api\UserApi;
+use PHPUnit\Framework\TestCase;
+
+class UserApiTest extends TestCase
+{
+
+    /** @var UserApi*/
+    private $api;
+
+    public function setUp(): void
+    {
+        $this->api = new Api\UserApi();
+    }
+
+    // test login use
+    public function testLoginUser()
+    {
+        // initialize the API client
+        // login
+        $response = $this->api->loginUser('xxxxx', 'yyyyyyyy');
+        
+        $this->assertIsString($response);
+
+        $pattern = '/logged in user session/';
+        $assertMessage = "response string starts with 'logged in user session'";
+        $this->assertIsString($response);
+        if (method_exists($this, 'assertMatchesRegularExpression')) {
+            $this->assertMatchesRegularExpression($pattern, $response, $assertMessage);
+        } else {
+            $this->assertRegExp($pattern, $response, $assertMessage);
+        }
+    }
+}