create_post_with_media.php

<?php
$client = new \GuzzleHttp\Client([
    'base_uri' => 'https://example.com/wp-json/wp/v2/'
]);
$client->post('posts',[
    'auth' => [$user, $password],
    'form_params' => [
        'title'          => 'WP REST API Example',
        'content'        => '<p>サンプル記事。Guzzle使用</p>',
        'date'           => '2019-07-10T20:00:00+09:00',
        'status'         => 'publish',
        'featured_media' => 1234
    ],
]);