Call to undefined function mb_substitute_character()

[nvjkmr]

I'm getting the following error:
Fatal error: Uncaught Error: Call to undefined function mb_substitute_character() in /var/www/html/my-app/application/bootstrap.php:64 Stack trace: #0 /var/www/html/my-app/index.php(102): require() #1 {main} thrown in /var/www/html/my-app/application/bootstrap.php on line 64

How to fix this error?

[nvjkmr]

Make sure you have compiled your PHP code with the --enable-mbstring option or just do sudo apt-get install php5.6-mbstring on Ubuntu.