src/AbstractHttpAdapter.php
public function getProtocolVersion()
    {
        if ($this->protocolVersion) {
            return $this->protocolVersion;
        }

        $protocolAndVersion = $_SERVER['SERVER_PROTOCOL'];
        list($protocol, $version) = explode('/', $protocolAndVersion);
        return $version;
    }
public function getProtocolVersion()
    {
        if ($this->protocolVersion) {
            return $this->protocolVersion;
        }

        $protocolAndVersion = $_SERVER['SERVER_PROTOCOL'];
        list($protocol, $version) = explode('/', $protocolAndVersion);
        return $version;
    }
Get the protocol version - either from a previously set value, or from the server

@return string E.g. "1.1"