function is_php($version)
        {
            static $_is_php;
            $version = (string)$version;
    
            if(!isset($_is_php[$version]))
            {
                $_is_php[$version] = version_compare(PHP_VERSION,$version,'>=');
            }
            return $_is_php[$version];
        }

这里为什么version_compare(PHP_VERSION,$version,'>=')不能直接赋值给 $_is_php而是赋值给 $_is_php[$version]这个呢？