使用Url::to('index/msg) 生成路径信息时的问题 [ 2.0 版本 ]

vendor/yiisoft/yii2/helpers/BaseUrl.php
这儿有说明。
例如site/index site是控制器名，index是行为名。

    /**
     * Creates a URL based on the given parameters.
     *
     * This method is very similar to [[toRoute()]]. The only difference is that this method
     * requires a route to be specified as an array only. If a string is given, it will be treated as a URL.
     * In particular, if `$url` is
     *
     * - an array: [[toRoute()]] will be called to generate the URL. For example:
     *   `['site/index']`, `['post/index', 'page' => 2]`. Please refer to [[toRoute()]] for more details
     *   on how to specify a route.
     * - a string with a leading `@`: it is treated as an alias, and the corresponding aliased string
     *   will be returned.
     * - an empty string: the currently requested URL will be returned;
     * - a normal string: it will be returned as is.
     *
     * When `$scheme` is specified (either a string or true), an absolute URL with host info (obtained from
     * [[\yii\web\UrlManager::hostInfo]]) will be returned. If `$url` is already an absolute URL, its scheme
     * will be replaced with the specified one.
     *
     * Below are some examples of using this method:
     *
     * ```php
     * // /index.php?r=site/index
     * echo Url::to(['site/index']);
     *
     * // /index.php?r=site/index&src=ref1#name
     * echo Url::to(['site/index', 'src' => 'ref1', '#' => 'name']);
     *
     * // /index.php?r=post/index     assume the alias "@posts" is defined as "/post/index"
     * echo Url::to(['@posts']);
     *
     * // the currently requested URL
     * echo Url::to();
     *
     * // /images/logo.gif
     * echo Url::to('@web/images/logo.gif');
     *
     * // images/logo.gif
     * echo Url::to('images/logo.gif');
     *
     * // http://www.example.com/images/logo.gif
     * echo Url::to('@web/images/logo.gif', true);
     *
     * // https://www.example.com/images/logo.gif
     * echo Url::to('@web/images/logo.gif', 'https');
     * ```
     *
     *
     * @param array|string $url the parameter to be used to generate a valid URL
     * @param boolean|string $scheme the URI scheme to use in the generated URL:
     *
     * - `false` (default): generating a relative URL.
     * - `true`: returning an absolute base URL whose scheme is the same as that in [[\yii\web\UrlManager::hostInfo]].
     * - string: generating an absolute URL with the specified scheme (either `http` or `https`).
     *
     * @return string the generated URL
     * @throws InvalidParamException a relative route is given while there is no active controller
     */
    public static function to($url = '', $scheme = false)
    {
        if (is_array($url)) {
            return static::toRoute($url, $scheme);
        }

        $url = Yii::getAlias($url);
        if ($url === '') {
            $url = Yii::$app->getRequest()->getUrl();
        }

        if (!$scheme) {
            return $url;
        }

        if (strncmp($url, '//', 2) === 0) {
            // e.g. //hostname/path/to/resource
            return is_string($scheme) ? "$scheme:$url" : $url;
        }

        if (($pos = strpos($url, ':')) == false || !ctype_alpha(substr($url, 0, $pos))) {
            // turn relative URL into absolute
            $url = Yii::$app->getUrlManager()->getHostInfo() . '/' . ltrim($url, '/');
        }

        if (is_string($scheme) && ($pos = strpos($url, ':')) !== false) {
            // replace the scheme with the specified one
            $url = $scheme . substr($url, $pos);
        }

        return $url;
    }

如果是路由，用Url::toRoute，它会根据urlManager里面配置的rules来生成url