Installed PHP-FPM long after installing PHP itself.
When executed, both of them said this:
php: error while loading shared libraries: libreadline.so.7: cannot open shared object file: No such file or directory
Solved this by updating libreadline (in my case from v6.3 to v7.0)
sudo pacman -S readline
And tada! Problem solved. No more errors.
CAUTION: This might break dependencies for bash, you might wanna make some symlinks named as older (.6.3, .6) versions for libreadline in /lib and similar links might be needed for libtinfo.