调用未定义的函数yii \ web \ mb_strlen()

I`m using Amazon server with Amazon Linux AMI. I install

yum install php55 php55-mysql php55-pdo

and went I run my project (with Yii2 framework) it give me

Call to undefined function yii\web\mb_strlen()

I try to install mb_strlen from here

yum install php-mbstring

but this form php5.3 and there is no for php5.5

How can I install mb_string for php5.5 ?

The problem isn't that the script is calling a function from the multibyte extension, it's the fact that the call is being namespaced to yii\web rather than in the global namespace, so PHP is searching for \yii\web\mn_strlen().

If you can identify where the call is made (e.g. through a stack dump or debug backtrace), it should be to \mb_strlen()