public function createHelpMessage()
    {
        $m  = 'Usage:' . PHP_EOL;
        $m .= '  -h,--help' . PHP_EOL;
        $m .= '    Display help message and exit.' . PHP_EOL;
        $m .= '  -p platform (required)' . PHP_EOL;
        $m .= '    Select platform [m]ac or [w]indows or [l]inux.' . PHP_EOL;
        $m .= '    Required except that "--help, -h" is specified.' . PHP_EOL;
        $m .= '  -d output_dir_path' . PHP_EOL;
        $m .= '    Enter the output directory path.' . PHP_EOL;
        $m .= '    If not specified, it is output to the path specified by .env.dafault|.env(DEFAULT_OUTPUT_DIR).' . PHP_EOL;
        $m .= '    If there is still no value, it is output to "selenium-downloader/xxx".' . PHP_EOL;
        $m .= '  -s selenium-standalone-server_ver' . PHP_EOL;
        $m .= '    Enter the version of selenium-standalone-server. (e.g 3.8.1, 3.7(3.7.0)' . PHP_EOL;
        $m .= '    (Recommend version 3.8.1)' . PHP_EOL;
        $m .= '  -c ChromeDriver_ver' . PHP_EOL;
        $m .= '    Enter the version of ChromeDriver. (e.g 2.43' . PHP_EOL;
        $m .= '  -g geckodriver_ver' . PHP_EOL;
        $m .= '    Enter the version of GeckoDriver. (e.g 0.23(0.23.0), 0.20.1' . PHP_EOL;
        $m .= '  -i IEDriverServer_ver' . PHP_EOL;
        $m .= '    Enter the version of IEDriverServer. (e.g 3.14(3.14.0)' . PHP_EOL;
        $m .= '  -b bit_of_os' . PHP_EOL;
        $m .= '    Enter the number of OS bits (32 or 64).' . PHP_EOL;
        $m .= '    Default is "32" (Because key input is earlier than 64bit version).' . PHP_EOL;
        $m .= '    Valid only when IEDriverServer is specified.' . PHP_EOL;
        $m .= PHP_EOL;
        $m .= 'e.g) 1 Basic.' . PHP_EOL;
        $m .= '$ php selenium_downloader.php -p m -s 3.8.1 -c 2.43 -g 0.23' . PHP_EOL;
        $m .= 'e.g) 2 When specifying the output directory.' . PHP_EOL;
        $m .= '$ php selenium_downloader.php -p m -d /your/path/to -s 3.8.1' . PHP_EOL;
        $m .= 'e.g) 3 When downloading the 64 bit version of the IEDriverServer.' . PHP_EOL;
        $m .= '$ php selenium_downloader.php -p w -i 3.14.0 -b 64' . PHP_EOL;
        $m .= 'e.g) 4 When downloading only geckodriver.' . PHP_EOL;
        $m .= '$ php selenium_downloader.php -p m -g 0.23' . PHP_EOL;
        $m .= 'or' . PHP_EOL;
        $m .= '$ php selenium_downloader.php -p m -s "" -c "" -g 0.23' . PHP_EOL;

        return $m;
    }