PHP: erreur undefined function mb_strtolower()

Le journal de log indique un message du type

AH01071: Got error 'PHP message: PHP Fatal error:  Call to undefined function mb_strtolower() in /path/to/file.php on line 666\n'

Installer le module mbstring (multibyte extension)

sudo apt-get install php5.6-mbstring
sudo systemctl restart php5.6-fpm.service

• https://stackoverflow.com/questions/21073747/fatal-error-call-to-undefined-function-mb-strtolower