src/Zipkin/Propagation/RequestHeaders.php
public function get($carrier, string $key): ?string
    {
        $lKey = strtolower($key);
        return $carrier->hasHeader($lKey) ? $carrier->getHeader($lKey)[0] : null;
    }
public function get($carrier, string $key): ?string
    {
        $lKey = strtolower($key);
        return $carrier->hasHeader($lKey) ? $carrier->getHeader($lKey)[0] : null;
    }
{@inheritdoc}

@param RequestInterface $carrier