vkext.so (a PHP extension)

vkext is a PHP extension that must be installed if you plan to use TL/RPC calls.
Without vkext, your code can be compiled, but TL/RPC won't work in plain PHP.

vkext exposes functions written in C because they are performance-critical (network layer).

If you don’t need TL/RPC, you don’t need vkext. You just need PHP polyfills, described earlier.

Check if vkext is installed

vkext for PHP 7.4 is installed on the php7.4-vkext package installation.

This package is listed in recommended of the kphp package, that's why you may have occasionally installed it.

How to check if vkext is installed:

php -m | grep vk_ext

If you see nothing, vkext is not installed.

Installing vkext

apt install php7.4-vkext

Installing vkext for a different PHP version

See Compiling KPHP from sources.