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.
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.
apt install php7.4-vkext
See Compiling KPHP from sources.