All XAMPP packages come with Multibyte String (php_mbstring.dll) extension installed.

If you have accidentally removed DLL file from php/ext folder, just add it back (get the copy from XAMPP zip archive - its downloadable).

If you have deleted the accompanying INI configuration line from php.ini file, add it back as well:

extension=php_mbstring.dll

Also, ensure to restart your webserver (Apache) using XAMPP control panel.

Additional Info on Enabling PHP Extensions

• install extension (e.g. put php_mbstring.dll into /XAMPP/php/ext directory)
• in php.ini, ensure extension directory specified (e.g. extension_dir = "ext")
• ensure correct build of DLL file (e.g. 32bit thread-safe VC9 only works with DLL files built using exact same tools and configuration: 32bit thread-safe VC9)
• ensure PHP API versions match (If not, once you restart the webserver you will receive related error.)

EDIT: Dylan Pierce has confirmed that you can already install some PHP 7.2 extensions in the same way you would for PHP 5. For mbstring in particular, you can execute:

sudo apt-get install php7.2-mbstring

To complement sparkmood's answer, this now works for PHP 7.2 if you already imported ondrej's PPA for it.

sudo apt-get install libapache2-mod-php7.2

Don't forget, either PHP 7 is available through ubuntu's official repositories or you will need an external PPA.