Error

PHP Fatal Erroryii\base\ErrorException

Uncaught Error: Call to undefined function yii\web\mb_strlen() in /home/sites/coop/www/hub/protected/vendor/yiisoft/yii2/web/ErrorHandler.php:404
Stack trace:
#0 /home/sites/coop/www/hub/protected/vendor/yiisoft/yii2/views/errorHandler/callStackItem.php(26): yii\web\ErrorHandler->argumentsToString(Array)
#1 /home/sites/coop/www/hub/protected/vendor/yiisoft/yii2/base/View.php(325): require('/home/sites/coo...')
#2 /home/sites/coop/www/hub/protected/vendor/yiisoft/yii2/base/View.php(247): yii\base\View->renderPhpFile('/home/sites/coo...', Array)
#3 /home/sites/coop/www/hub/protected/vendor/yiisoft/yii2/web/ErrorHandler.php(241): yii\base\View->renderFile('/home/sites/coo...', Array, Object(yii\web\ErrorHandler))
#4 /home/sites/coop/www/hub/protected/vendor/yiisoft/yii2/web/ErrorHandler.php(295): yii\web\ErrorHandler->renderFile('@yii/views/erro...', Array)
#5 /home/sites/coop/www/hub/protected/vendor/yiisoft/yii2/views/errorHandler/exception.php(385): yii\web\ErrorHandler->renderCallStackItem('/home/sites/coo..

$_GET = [
    'r' => 'calendar/view/index',
    'sguid' => 'ffccc5d0-d20b-4d8b-bbd5-eccb754acae4',
];