This is sample application which is running on Heroku. Pure Heroku php doesn't support mbstring.
This sample is challenging to use mbstring as shared library.
heroku run bash
allows to run command on heroku.
I did it steps bellow same as mongodb practice.
mkdir tmp
cd tmp
git clone https://github.com/php/php-src.git
cd php-src
git checkout php-5.3.10
cd ext/mbstring
/app/php/bin/phpize
./configure --with-php-config=/app/php/bin/php-config
make