src/Router/Router.php
private function transformHeadersToServerVariables($headers)
    {
        $server = [];

        foreach ($headers as $headerType => $headerValue) {
            $headerType = 'HTTP_' . $headerType;

            $server[$headerType] = $headerValue;
        }

        return $server;
    }
private function transformHeadersToServerVariables($headers)
    {
        $server = [];

        foreach ($headers as $headerType => $headerValue) {
            $headerType = 'HTTP_' . $headerType;

            $server[$headerType] = $headerValue;
        }

        return $server;
    }
https://github.com/symfony/symfony/issues/5074

@param  array $headers
@return array