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
/XAMPP/php/ext
directory)extension_dir = "ext"
)