I have a very strange problem.

I use: Ubuntu 16.04.6 LTS
I have two running versions of php: php5 and php7.0

I want to install php7.0-xml but when I type in the following command:

apt-get install php7.0-xml

I get this:

Reading package lists... Done
Building dependency tree... 50%
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:
 php7.0-xml : Depends: php7.0-common (= 7.0.33-1~dotdeb+8.1) but 7.0.33-6+ubuntu14.04.1+deb.sury.org+3 is to be installed
E: Unable to correct problems, you have held broken packages.

I do this:

sudo apt-get install php7.0-common

and I get this:

Reading package lists... Done
Building dependency tree
Reading state information... Done
php7.0-common is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 7 not upgraded.

I am very confused. How php7.0-common is installed but php7.0-xml does not want to be installed? How can I fix that?