public static function os()
    {
        if ( is_null( self::$os ) )
        {
            $uname = php_uname( 's' );
            if ( substr( $uname, 0, 7 ) == 'Windows' )
            {
                self::$os = 'Windows';
            }
            elseif ( substr( $uname, 0, 3 ) == 'Mac' )
            {
                self::$os = 'Mac';
            }
            elseif ( strtolower( $uname ) == 'linux' )
            {
                self::$os = 'Linux';
            }
            elseif ( strtolower( substr( $uname, 0, 7 ) ) == 'freebsd' )
            {
                self::$os = 'FreeBSD';
            }
            else
            {
                self::$os = PHP_OS;
            }
        }
        return self::$os;
    }