upload_media.php

<?php
$client = new \GuzzleHttp\Client();
$response = $client->get('https://example.com/sample.jpg');
$image = $response->getBody()->getContents();

$client = new \GuzzleHttp\Client([
    'base_uri' => 'https://example.com/wp-json/wp/v2/'
]);
$response = $client->post('media',[
    'auth' => [$user, $password],
    'headers' => [
        'Content-Disposition' => [
            'form-data; filename=sample.jpg'
        ]
    ])
    ,'multipart' => [
        [
            'name'     => 'file',
            'contents' => $image,
            'filename' => 'sample.jpg',
        ],
    ]
]);

if(preg_match('/(\d+)$/',$response->getHeader('Location')[0],$matches)){
    $media_id = (int) $matches[1];
}