This error tells us that the mb_string library is not installed. mb_String is a library, which provides support for UTF-8 and UCS-2 characters. PHP uses ASCII by default which is a 256 character coding system. This means that it uses 8 bits to encode English alphabet and commonly used symbols such as ,.?;:”+-$#@ and many more. However, it does not code for foreign language characters such as é, ç, â, è, ñ, ü, and many others. mb_string provides double byte, i.e. 2 x 8bits to code for a character. This permits code for 65536 characters.
To remove this error, simply install the library, and add its extension to php.ini file.
Installing mb_string on Ubuntu, Debian systems
$ sudo apt-get install php-mbstring
This command install the library and configures the php.ini file.
Installing mb_string on Red Hat family
$ sudo yum install php-mbstring
This command install the library and configures the php.ini file.
Installing mb_string on Microsoft Windows As always, all non-click tasks are complicated in Microsoft systems. For information on installing and configuring mb_string on Microsoft Windows, visit http://php.net/manual/en/install.windows.extensions.php.