Gfilui

Try to install php-mbstring for mediawiki setup.

already added after first attempt universe repository: Package php7.2-mbstring missing in Ubuntu 18.04

But gives me hard time
Here details:

usertilo@myserver4:~$ sudo apt-get install php-mbstring
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 php-mbstring : Depends: php7.2-mbstring but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

usertilo@myserver4:~$ sudo apt-get install php7.2-mbstring
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 php7.2-mbstring : Depends: php7.2-common (= 7.2.3-1ubuntu1) but 7.2.7-0ubuntu0.18.04.2 is to be installed
E: Unable to correct problems, you have held broken packages.

usertilo@myserver4:~$ uname -a
Linux myserver4 4.15.0-29-generic #31-Ubuntu SMP Tue Jul 17 15:39:52 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

usertilo@myserver4:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.1 LTS
Release: 18.04
Codename: bionic

usertilo@myserver4:~$ sudo apt-get install -f
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

usertilo@server4:~$ sudo apt-get install php7.2-common
Reading package lists... Done
Building dependency tree
Reading state information... Done
php7.2-common is already the newest version (7.2.7-0ubuntu0.18.04.2).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

• update add info requested from comments

usertilo@myserver4:~$ sudo apt-cache policy php7.2-common php7.2-mbstring
php7.2-common:
 Installed: 7.2.7-0ubuntu0.18.04.2
 Candidate: 7.2.7-0ubuntu0.18.04.2
 Version table:
 *** 7.2.7-0ubuntu0.18.04.2 500
 500 http://archive.ubuntu.com/ubuntu bionic-security/main amd64 Packages
 500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
 100 /var/lib/dpkg/status
 7.2.3-1ubuntu1 500
 500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages
php7.2-mbstring:
 Installed: (none)
 Candidate: 7.2.3-1ubuntu1
 Version table:
 7.2.3-1ubuntu1 500
 500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages

usertilo@myserver4:~$ cat /etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu bionic main universe
deb http://archive.ubuntu.com/ubuntu bionic-security main
# deb-src http://archive.ubuntu.com/ubuntu bionic universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main

usertilo@myserver4:~$ cat /etc/apt/sources.list.d/*
cat: '/etc/apt/sources.list.d/*': No such file or directory

Your package information that apt knows about is out of date.

Using the rmadison tool, we can see all the versions of php7.2-mbstring in the repos:

 php7.2-mbstring | 7.2.3-1ubuntu1 | bionic/universe | amd64, arm64, armhf, i386, ppc64el, s390x
 php7.2-mbstring | 7.2.7-0ubuntu0.18.04.2 | bionic-security/universe | amd64, arm64, armhf, i386, ppc64el, s390x
 php7.2-mbstring | 7.2.7-0ubuntu0.18.04.2 | bionic-updates/universe | amd64, arm64, armhf, i386, ppc64el, s390x

though I am only showing the Bionic items here.

Because your system doesn't know about the newer php7.2-mbstring in the security or updates repos, you're getting a version mismatch.

Run this:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install php7.2-mbstring

This should assist in getting your versions back to where they need to be. (Note you may wish to use dist-upgrade instead of upgrade so that packages such as updated kernel packages get installed and upgraded too)

You are missing some necessary channels from sources.list. Do sudo gedit /etc/apt/sources.list and make it look like this:

deb http://archive.ubuntu.com/ubuntu bionic main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-security main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main multiverse restricted universe

Save and exit.
Then update your repository:

sudo apt update

Then install php-mbstring:

sudo apt install php7.2-mbstring