Call to undefined function Typecho\mb_strlen()

Error: Call to undefined function Typecho\mb_strlen() in /var/www/junning.live/var/Typecho/Common.php:686 Stack trace: #0 /var/www/junning.live/var/Typecho/Common.php(671): Typecho\Common::strLen() #1 /var/www/junning.live/var/Widget/Base/Contents.php(860): Typecho\Common::subStr() #2 /var/www/junning.live/var/Widget/Archive.php(1841): Widget\Base\Contents->___description() #3 /var/www/junning.live/var/Widget/Archive.php(739): Widget\Archive->singleHandle() #4 /var/www/junning.live/var/Typecho/Widget.php(155): Widget\Archive->execute() #5 /var/www/junning.live/var/Typecho/Router.php(96): Typecho\Widget::widget() #6 /var/www/junning.live/index.php(23): Typecho\Router::dispatch() #7 {main}