Call to undefined function 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 ?