The function mb_strlen() is not enabled by default in PHP. Please read the manual for installation details:

http://www.php.net/manual/en/mbstring.installation.php

For PHP 4.3.3 or before, To enable that feature, you will have to supply either one of the following options to the LANG parameter of --enable-mbstring=LANG ; --enable-mbstring=cn for Simplified Chinese support, --enable-mbstring=tw for Traditional Chinese support, --enable-mbstring=kr for Korean support, --enable-mbstring=ru for Russian support, and --enable-mbstring=ja for Japanese support (default). To enable all supported encoding, use --enable-mbstring=all

try

• configure again with --enable-mbstring only

• grep mb_strlen $PATH_TO_YOUR_PHP_BINARY to see is it exist

The error is a bit of a red herring since mb_strlen() is actually part of the mbstring php extension and not a function in yii2.

In php5 mbstring was part of libapache2-mod-php5.

It seems to be missing from libapache2-mod-php7. To fix this install the php7.x-mbstring package where x is your version:

sudo apt install php7.x-mbstring

You may need to restart Apache after this install.

After over 3,5 hours of search, I found the solution!!! I HOPE IT WILL HELP SOMEONE ELSE !!

THE ERROR

So, the problem was that the extension_dir in the php.ini file was set to the wrong file. By default, it was to the following:

extension_dir = './'

THE SOLUTION

1. Open you terminal ctrl+alt+t and type:

php -i | grep extension_dir

In my case, it ouputs:

/user/lib/php/20180731

2. Find out your php.ini file: You can do that either by:

<?php
phpinfo();

Or on the terminal

php -i | grep 'php.ini'

3. Go to your php.ini file and change the following line:

extension_dir = '/full/path/to/your/php/modules'

in my case, it was:

exension_dir = '/usr/lib/php/20180731'

SEE MORE

By the way, you have to do this after installing mbstring. To do that, You may find useful the following links:

PHP: Call to undefined function mb_strlen() - on custom compiled PHP with mbstring enabled

Fatal error: Call to undefined function mb_strlen()

Call to undefined function yii\helpers\mb_strlen() on PHP in Yii2 Framework

https://askubuntu.com/questions/772397/mbstring-is-missing-for-phpmyadmin-in-ubuntu-16-04

If you really have to use mb_-functions, you have to check if the multibyte string extension is installed on your system.

Check if the extension is installed?

<?php phpinfo(); ?>

Extension is not displayed?

Install the extension:

But therefore you have to reveal on which OS your PHP is running. So for example, in Ubuntu this can be achieved by running one of the following commands:

sudo apt-get install php-mbstring
sudo apt-get install php7.0-mbstring
sudo apt-get install php7.1-mbstring

Of course, this depends on the installed version of PHP.

Don´t forget to restart PHP after the extension was installed.

CLI configuration is separate from web configuration. Depending on your environment you have to change the php.ini file that is being used when PHP is accessed via whatever web server you use.

In order to check what is the location of the php.ini file in use, you can run phpinfo() from your web app (not CLI). The phpinfo() output will have information of php.ini file location.