To install the PHP mbstring extension on Alpine Linux, you need to follow a few steps. Alpine Linux uses apk for package management. 

Update the package list:
Open your terminal and update the package list to ensure you have the latest information on available packages.

sudo apk update


Install PHP and the required extensions:
You need to install PHP and the mbstring extension. The package for mbstring is php7-mbstring if you're using PHP 7, or php8-mbstring for PHP 8.
# For PHP 7

apk add php7 php7-mbstring



# For PHP 8

apk add php8 php8-mbstring


Verify the installation:
Check if the mbstring extension is enabled by creating a PHP file (e.g., info.php) and adding the following code:
<?php phpinfo(); ?>

Save this file in your web server's document root (e.g., /var/www/html), then access it via your web browser (e.g., http://your-server-ip/info.php). Look for a section labeled "mbstring" to confirm that the extension is installed and enabled.

Restart your web server:
After installing the extension, you might need to restart your web server to apply the changes. For example, if you are using Apache, you can restart it with:
sudo service apache2 restart


For Nginx, you would use:
sudo service nginx restart


Check PHP CLI configuration:
If you need to use the mbstring extension in the PHP CLI, you can check if it is enabled by running:
php -m | grep mbstring

If mbstring appears in the output, the extension is successfully installed and enabled for the PHP CLI.

By following these steps, you should have the PHP mbstring extension installed and configured on Alpine Linux.