src/Firebase.php
public function post($endpoint, $data, $query = [])
    {
        $endpoint = $this->buildUri($endpoint, $query);
        $headers = $this->buildHeaders();
        $data = $this->prepareData($data);

        $this->response = $this->client->post($endpoint, $data, $headers);

        return $this->response;
    }
public function post($endpoint, $data, $query = [])
    {
        $endpoint = $this->buildUri($endpoint, $query);
        $headers = $this->buildHeaders();
        $data = $this->prepareData($data);

        $this->response = $this->client->post($endpoint, $data, $headers);

        return $this->response;
    }
POST request.

@param string       $endpoint The sub endpoint
@param string|array $data     The data to be submited
@param array        $query    Query parameters

@return object