In T10586#179056, @storrgie wrote:

@valentinul can you please let me know what exactly you modified in your php.ini?

OK, after installing all the necessary extensions and changing the php.ini as recommended by phabricator everything works nice.

OK, I'm going to switch to php7 and see if it goes.

@epriestley that is from PHP 5.6.17

The issue is not comming from missing php-extentions. mysqli was installed and present in fpm and cli so that was not the issue.
The actual function is missing, as @eadler said in the previous comment.