example.php

<?php

use GuzzleHttp\Client;

$client = new Client();

$client->request('POST', 'http://localhost:8080/ag-push/rest/applications/{PUSH_ID}/iOS', [
  'debug' => true,
  'headers' => [
    'Authorization' => 'Bearer OAUTHTOKEN',
    'Accept' =>  'application/json'
    'Content-type' => 'multipart/form-data',
    
  ],
  'multipart' => [
    [
      'name' => 'production',
      'contents' => 'true',
    ],
    [
      'name' => 'passphrase',
      'contents' => 'passw0rd',
    ],
    [
      'name' => 'certificate',
      'contents' => fopen('/Users/matzew/Desktop/MyProdCert.p12', 'r'),
    ],
  ],
]);

headers.txt

User-Agent: GuzzleHttp/6.1.1 curl/7.47.1 PHP/5.6.19
Content-Type: multipart/form-data; boundary=56e97c6435a37
Authorization: Bearer OAUTHTOKEN
Content-Length: 1838

* upload completely sent off: 1838 out of 1838 bytes
< HTTP/1.1 400 Bad Request
< Date: Wed, 16 Mar 2016 15:31:49 GMT
< Server: WildFly/8
< Expires: 0
< Cache-Control: no-cache, no-store, must-revalidate
< X-Powered-By: Undertow/1
< Pragma: no-cache
< Content-Type: text/html
< Content-Length: 80
< Connection: close
<