guzzle.php

$response = $client->post('http://example.com/api', [
    'multipart' => [
        [
            'name'     => 'document',
            'filename' => <filename>
            'contents' => "@$path_to_file"
        ],
        [
            'name'     => 'metadata',
            'headers'  => ['Content-Type' => 'application/json']
            'contents' => ['document' =>  [
                “filename” => <filename>,
                 “size” : <filesize>,
                 “title” : <title>,
                 “author” : <author>,
                 “document-date” : <document-date>
              ]
          ]
        ]
    ]
]);