I tried a different installation way.
$ curl -s https://packagecloud.io/install/repositories/phalcon/stable/script.deb.sh | sudo bash
Detected operating system as LinuxMint/tara.
Checking for curl...
Detected curl...
Checking for gpg...
Detected gpg...
Running apt-get update... done.
Installing apt-transport-https... done.
Installing /etc/apt/sources.list.d/phalcon_stable.list...done.
Importing packagecloud gpg key... done.
Running apt-get update... done.
The repository is setup! You can now install packages.
But after updating the repository, I get an error.
$ sudo apt update
...
Hit:19 https://repo.skype.com/deb stable InRelease
Ign:20 https://packagecloud.io/phalcon/stable/linuxmint tara InRelease
Err:22 https://packagecloud.io/phalcon/stable/linuxmint tara Release
404 Not Found [IP: 52.8.147.134 443]
Reading package lists... Done
E: The repository 'https://packagecloud.io/phalcon/stable/linuxmint tara Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.