<?php
$client = new http\Client;
$request = new http\Client\Request;
$request->setRequestUrl('[your-url]/api/3.0/workflows/data-set/51');
$request->setRequestMethod('GET');
$body = new http\Message\Body;
$body->append('{
  "workflow_id": 41,
    "contact_email":  "[email protected]",
    "dsv_41_flight_date": "2020-10-04",
    "dsv_41_flight_number": "2020-10-04"
}');
$request->setBody($body);
$request->setOptions(array());
$request->setHeaders(array(
  'Authorization' => 'Basic ',
  'Content-Type' => 'application/json',
  'Authorization' => 'Basic ',
  'Content-Type' => 'text/plain'
));
$client->enqueue($request)->send();
$response = $client->getResponse();
echo $response->getBody();

Success Result: 

{
    "result": "success",
    "data": [
        {
            "ds_name": "Reservation Date",
            "ds_sanitised_name": "dsv_51_reservation_date",
            "ds_is_required": "1",
            "ds_is_primary": "1",
            "ds_type": "date"
        },
        {
            "ds_name": "Flight Number",
            "ds_sanitised_name": "dsv_51_flight_number",
            "ds_is_required": "0",
            "ds_is_primary": "0",
            "ds_type": "integer"
        },
        {
            "ds_name": "City",
            "ds_sanitised_name": "dsv_51_city",
            "ds_is_required": "1",
            "ds_is_primary": "0",
            "ds_type": "varchar"
        }
    ]
}

Error 

{
    "result": "error",
    "data": {
        "messages": [
            "Workflow does not exist"
        ]
    }
}