class OptimusClient {

        private $merchantId;
        private $apiKey;
        private $baseUrl;
        private $guzzleClient;
    
        public function __construct($merchantId, $apiKey, $baseUrl)
        {
            $this->merchantId = $merchantId; // YOUR_MERCHANT_ID
            $this->apiKey = $apiKey; // YOUR_API_KEY

            // https://optimus-staging.getcarbon.co
            $this->baseUrl = $baseUrl;

            // OR ANY OTHER HTTP CLIENT
            $this->guzzleClient = new GuzzleHttp\Client();
        }
    
    
        public function makeOptimusTransferApiRequest ($requestObject = []) {
            try {
                /**
                  * If Request doesnt require a body e.g GET Request then requestRawBody = "";
                  */
                $requestRawBody = json_encode($requestObject);
                $dateObject = new DateTime("now", new DateTimeZone('UTC'));
    
                // Format Sun, 25 Mar 2019 10:21:49 GMT
                $dateHeaderValue = date_format($dateObject,'D, d M Y H:i:s \G\M\T');
    
                $bodyDigest = hash("sha256", $requestRawBody);
                $digestHeaderValue = 'SHA-256='.$bodyDigest;
    
                // `${REQUEST_METHOD} ${REQUEST_PATH} HTTP/1.1`
                $requestLine = "POST /api/v2/transfers/sendmoney HTTP/1.1";
                          
                $signingString = join("/n", [
                    "date: $dateHeaderValue",
                    "digest: $digestHeaderValue",
                    "host: $this->baseUrl",
                    $requestLine
                ]);
    
                $signature = hash_hmac("sha256", $signingString, $this->apiKey);
                $hMACAuth = 'hmac '.$this->merchantId.':'.$signature;
    
                $response = $this->guzzleClient->post($this->baseUrl.'/api/v2/transfers/sendmoney', [
                    'headers' => [
                        'Date' =>  $dateHeaderValue,
                        'Digest' => $digestHeaderValue,
                        'Authorization'=> $hMACAuth
                    ],
                    'json' => $requestObject
                ]);
                return $response->getBody();
    
            } catch (\Exception $e) {
                /**
                  * Handle Exception
                  */
            }
        }
      }
              
            

                
        package co.getcarbon.optimus.internal.client;
        // Imports skipped for Brevity

        public class OptimusApiGateway {

          private String merchantId;
      
          private String apiKey;
      
          private String hostName;
      
          public OptimusApiGateway(String merchantId, String apiKey) {
              this.merchantId = merchantId;
              this.apiKey = apiKey;
              hostName = "https://optimus-staging.getcarbon.co";
          }
      
          public Object doSingleTransfer (Object transferRequest) throws Exception{
      
              try {
                  Gson gson = new Gson();
      
                  /**
                    * Convert Object to JSON String
                    * For requests do not require a body e.g GET requests, rawRequestBody should be "" (empty string)
                    */
                  String rawRequestBody = gson.toJson(transferRequest);
      
                  DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("E, dd MMM yyyy HH:mm:ss");
                  ZoneOffset zoneOffSet = ZoneOffset.of("+00:00");
                  String dateHeaderValue = dateTimeFormatter.format(LocalDateTime.now(zoneOffSet)) + " GMT";

                  String bodyDigest = Base64.getEncoder().encodeToString(DigestUtils.sha256(rawRequestBody));
                  String digestHeaderValue = "SHA-256=" + bodyDigest;
      
                  /**
                    * `${REQUEST_METHOD} ${REQUEST_PATH} HTTP/1.1`;
                    */
                  String transferEndpoint = "/api/v2/transfers/sendmoney";
                  String requestLine = String.format("POST %s HTTP/1.1", transferEndpoint);
                  String signingString = String.join(
                          "\n",
                          Arrays.asList(
                              "date: " + dateHeaderValue,
                              "digest: " + digestHeaderValue,
                              "host: optimus-staging.getcarbon.co",
                              requestLine
                          )
                  );
      
                  /**
                    * Do HMAC Encoding
                    */
                  Mac sha256 = Mac.getInstance("HmacSHA256");
                  SecretKeySpec secretKeySpec = new SecretKeySpec(apiKey.getBytes(StandardCharsets.UTF_8), "HmacSHA256");
                  sha256.init(secretKeySpec);
      
                  String signature = Base64.getEncoder().encodeToString(sha256.doFinal(signingString.getBytes(StandardCharsets.UTF_8)));
      
                  /**
                    * Generate HMAC Authorization String
                    */
                  String hMACAuth = String.format("hmac %s:%s", merchantId, signature);

                  RestTemplate restTemplate = new RestTemplate();
      
                  HttpHeaders httpHeaders = new HttpHeaders();
                  httpHeaders.set("Accept", "application/json");
                  httpHeaders.set("Content-Type", "application/json");
                  httpHeaders.set("Date", dateHeaderValue);
                  httpHeaders.set("Digest", digestHeaderValue);
                  httpHeaders.set("Authorization", hMACAuth);
      
                  HttpEntity<Object> httpEntity = new HttpEntity<>(transferRequest, httpHeaders);
                  ResponseEntity<String> responseJson = restTemplate.exchange(hostName + transferEndpoint, HttpMethod.POST, httpEntity, String.class);
                  return gson.fromJson(responseJson.getBody(), Object.class);
              } catch (Exception e){
                  // Handle Exception
                  throw new Exception("Unknown Error");
              }
          }
      }