Laravel 5.1 : FatalErrorException in Encrypter.php line 48: Call to undefined function Illuminate\Encryption\mb_strlen()

I installed laravel 5.1 in my server. I am getting fatal error like in laravel 5.1 Call to undefined function Illuminate\Foundation\Bootstrap\mb_internal_encoding() then i commented the line mb_internal_encoding('UTF-8'); in LoadConfiguration.php..

again i am getting one more fatal error

Laravel 5.1 : FatalErrorException in Encrypter.php line 48: Call to undefined function Illuminate\Encryption\mb_strlen() 

can anybody tell me how to resolve this?

via Chebli Mohamed