Here is sample code for sending single document...
$client = new SoapClient(EchoSign\API::getWSDL());
$api = new EchoSign\API($client, 'YOUR API KEY');
$file = EchoSign\Info\FileInfo::createFromFile('demo.pdf');
$document = new EchoSign\Info\DocumentCreationInfo('Test sendDocument', $file);
$recipients = new EchoSign\Info\RecipientInfo;
$result = $api->sendDocument($document);
Please find the documentation of DocumentCreationInfo object in EchoSign SOAP APIs at https://secure.echosign.com/public/docs/EchoSignDocumentService22#DocumentCreationInfo
You will see that you can provide an array of files in the fileInfos field of DocumentCreationInfo object.
EchoSign also provides REST style APIs. You can access the documentation of REST APIs at https://secure.echosign.com/public/docs/restapi/v5. You can use POST /agreements API https://secure.echosign.com/public/docs/restapi/v5#!/agreements/ to send the agreement and this API operation also accepts multiple files in the request.
Hope this helps.