Server-sent events in PHP (without echo or print)

PHP Snippet 1:

   /**
     * Send Data
     *
     * @param string $content
     */
    function sseEchoAlternative(string $content)
    {
        ob_start(function () use ($content) { //Expect a warning here
            return $content;
        });
        ob_end_flush();
    }