If you are using ubuntu you can take update
$ sudo apt-get update
And install extension in case of php 5.6
$ sudo apt-get install php5.6-intl
And in case of php 7.0
$ sudo apt-get install php7.0-intl
And in case of php 8.1
$ sudo apt-get install php8.1-intl
And restart your apache after
$ sudo service apache2 restart
And restart your nginx after
$ sudo systemctl restart nginx
If you are using xampp then remove semicolon ( ; ) in xampp/php/php.ini from below line
;extension=php_intl.dll