Install dependencies
Add respositores
sudo apt-add-repository ppa:ondrej/php
Install
sudo apt-get install php7.3-dev libpcre3-dev gcc make
You can clone a specific tag using this command:
git clone --branch v3.4.5 --depth=1 "git://github.com/phalcon/cphalcon.git"
and now build the extension
cd cphalcon/build
sudo ./install
You will now need to add extension=phalcon.so to your PHP ini and restart your web server, so as to load the extension.
extension=phalcon.so