What is this?
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.
What I did
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