Detect SSL in PHP

if (!isset($_SERVER['HTTPS']) || $_SERVER['HTTPS'] !== 'on') {
    if(!headers_sent()) {
        header("Status: 301 Moved Permanently");
        header(sprintf(
            'Location: https://%s%s',
            $_SERVER['HTTP_HOST'],
            $_SERVER['REQUEST_URI']
        ));
        exit();
    }
}