centos PHP7 yii2 报错 Call to undefined function yii\helpers\mb_strlen()

出现问题 

Call to undefined function yii\helpers\mb_strlen()

    "data": {
        "name": "Exception",
        "message": "Call to undefined function yii\helpers\mb_strlen()",
        "code": 0,
        "type": "Error",
        "file": "............../vendor/yiisoft/yii2/helpers/BaseStringHelper.php",
        "line": 31,

解决,安装 PHP扩展 php-mbstring:

yum install php-mbstring
systemctl restart php-fpm