add php extension php-mbstring check

themes/tajian/controller/SiteController.php

@@ -8,6 +8,10 @@ require_once __DIR__ . '/../../../plugins/Parsedown.php';
 Class SiteController extends Controller {
 
     public function actionIndex() {
+        if (function_exists('mb_strlen') == false) {
+            throw new Exception('Please install php extension php-mbstring first!', 500);
+        }
+
         //获取数据
         $menus = array();        //菜单，一级目录
         $htmlReadme = '';   //Readme.md 内容，底部网站详细介绍