WARNING: Module ini file doesn't exist under /etc/php/7.0/mods-available

I came here because I had trouble installing Phpmyadmin on Ubuntu after I upgrade from php 7.0 to 7.1

Here is the tricks

1) remove php version (7.0 for instance - adapt the command line)

sudo apt-get purge php7.0-common

Note --> You should check the list of packaged that will be removed.

2) Install php modules

sudo apt-get install php7.1-mbstring
sudo apt-get install php7.1-mcrypt
...

3) activate the modules

sudo phpenmod mbstring
sudo phpenmod mcrypt

(option for apache) activate php7.1

sudo a2enmod php7.1

And of course restart your server (apache in this case)

sudo systemctl restart apache2

The problem was because of deleting php with sudo apt-get remove [package] and with this form, configuration files won't clean from computer and the next time you try to install the package, Ubuntu won't let the package configuration files to overwrite. the solution is to delete the package completely with purge and the autoremove the package so that It will completely remove from the computer with its configuration files and the next time you try to install, the configuration files with be created.

For me, doing this single command solved the problem:

sudo apt install php7.0-mbstring

I noticed that sometimes (I don't know when and why) the module is called without the php version (e.g. sudo phpenmod mbstring) but php search it by the php's version in use (in my case is the 7.0.33).

I'm using Ubuntu 18.04, just installed 1 our ago.