“即使在成功安装apt-get之后，系统也缺少”请求的PHP扩展mbstring“

Question

I'm trying to

$composer require --dev phpunit/phpunit ^6.5

and get the following error:

 Problem 1
    - phpunit/phpunit 6.5.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

So I ran the following multiple times

$sudo apt-get install php-mbstring

and get

php-mbstring is already the newest version (1:7.2+60ubuntu1).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

$ php --ini

Gives:

Configuration File (php.ini) Path: /etc/php/7.0/cli
Loaded Configuration File:         /etc/php/7.0/cli/php.ini

I've also uncommitted the extension=php_mbstring.dll line in /etc/php/7.0/cli/php.ini (irrelevant/wrong action)

And I still get the "mbstring is missing from your system" error.

I'm on Ubuntu 18.04

EDIT: the composer command also gives:

PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/php_mbstring.dll' - /usr/lib/php/20151012/php_mbstring.dll: cannot open shared object file: No such file or directory in Unknown on line 0

Yes, un-commenting extension=php_mbstring.dll in php.ini on ubuntu was wrong/irrelevant (see comment)