The problem came from a different mediawiki version (oups didn't checked that). I installed through:
sudo apt-get install mediawiki
But the version on the repository was 1.15 and not 1.25.
The video did help as I realised this gap in version within the first few images "Download mediawiki 1.24.2". So thank you for pointing toward it. I didn't notice there was some in the "Help:Installation" page.
I take the liberty to add a "version checking " line in the steps describe. May it help those hasty and inattentive as I can sometime be.