Guzzle ~6.0 multipart and form_params
I ran into the same problem. You need to add your form_params to the multipart array. Where 'name' is the form element name and 'contents' is the value. The example code you supplied would become:
$response = $client->post('http://example.com/api', [ 'multipart' => [ [ 'name' => 'image', 'contents' => fopen('/path/to/image', 'r') ], [ 'name' => 'name', 'contents' => 'Example name' ] ]]);
I got there too, but unfortunately it does not work if you have multidimensional params array. The only way i got it to work is if you send the form_paramaters as query parameters in the array:
$response = $client->post('http://example.com/api', [ 'query' => [ 'name' => 'Example name', ], 'multipart' => [ [ 'name' => 'image', 'contents' => fopen('/path/to/image', 'r') ] ]]);