admin/environment/php extension/xmlrpc

To install the xmlrpc library on Microsoft Windows:

1. Open the php.ini file, depending on your installation this may be found in the moodle/apache/bin folder
2. Find the line: ;extension=php_xmlrpc.dll
3. Remove the ; at the beginning of the line
4. If necessary restart the web server, e.g. IIS or Apache.

To install the xmlrpc library on Linux/Unix

If you are using PHP as provided by the OS, you can just install the appropriate package, and restart the service (e.g. PHP-FPM or Apache):

• On Ubuntu and Debian, the command is: apt-get install php-xmlrpc
• On RedHat, Fedora, CentOS and SuSE, the command is: dnf install php-xmlrpc

If you compiled your PHP from source:

1. You need to recompile PHP from source
2. add --with-xmlrpc to the command line when you run configure

You can (usually) ignore this warning

According to this post: There are 2 features in Moodle that require that PHP extension - both are disabled by default:

One called "Moodle networking" or "Mnet" - has been historically used for connecting multiple Moodle sites to each other so you can share login processes, or connecting Moodle to Mahara (an e-portfolio system)

The other feature allows for an external system to make webservice calls to your Moodle site - there are a few different ways that this data can be formatted, and one of those uses the xmlrpc php exension.

Both those features are slated for removal at some point as they mostly have supported altertnatives in Moodle, (and you aren't using the features anyway) so you can ignore the warning.