mediawikiセットアップ用のphp-mbstringをインストールしてみてください。
ユニバースリポジトリの最初の試行後にすでに追加されています：Ubuntu 18.04にパッケージphp7.2-mbstringがありません
しかし、私に苦労を与えますここの詳細：
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.- コメントからリクエストされた追加情報の更新
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
@TiloBuntどのソースにあります
—
ソクラテス
/etc/apt/sources.listか？そしての出典についても同じ質問です/etc/apt/sources.list.d/*。これらは、すべてのパッケージのソースです。だから、それがすべて問題なく見えることを確認してください。インストールするパッケージのリポジトリを取得し、そのソースが上記のリストにあるかどうかを確認します。
@ThomasWardとSocratesが質問に情報を追加しました。他に確認する必要があるかどうかをお知らせください。まったく新しいインストールです
—
TiloBunt 2018
apt-cache policy php7.2-common php7.2-mbstring（また、FYI、ユニバースはデフォルトで有効になっています） Ubuntu 18.04）