I have been running php 7.4 FPM with apache since it was released - today after doing an apt upgrade we started getting 'Uncaught Error: Call to undefined function xmlrpc_encode_request()' . After running a php -v we are now seeing PHP 8.0 as the active version of php. HOWEVER when looking at the output of phpinfo() in the actual browser - 7.4fpm was still serving pages BUT php-xmlrpc was no longer installed.
I found MANY articles describing a simple 'apt remove php-xmlrpc' then 'apt install php-xmlrpc' as the solution however this just gets me back to the start. It seems that php 8.0 hoses certain libraries and 'sort of' takes over.
It also appears that other libraries are missing as well such as php-xml. The problem is after running apt upgrade apt is suggesting only php 8.0 as a base php version and all 8.0 libraries?
The quick fix is to remove php 8.0 (although not required) and to then install the proper version of xmlrpc using 'apt-get install php7.4-xmlrpc' - this puts the proper version of xmlrpc back in place and allows php7.4 to run as expected.
sudo apt-get install php7.4-xml was also needed to fix the error Call to undefined function simplexml_load_string()