Summary

PHP-FPM, PHP FastCGI Process Manager, is a part of PHP package in OpenBSD packages nowadays.
So installing PHP (php-7.? due to the version) comes with php7?_fpm automatically 💃
I'll show you how to set it up in this post 😃

Environment

OS: OpenBSD 6.4 amd64
PHP: 7.2

✿ ✿ ✿

Installation

The first step is to install the PHP package:

# pkg_add php
Ambiguous: choose package for php
a       0: <None>
        1: php-5.6.38p0
        2: php-7.0.32p1
        3: php-7.1.22
        4: php-7.2.10
Your choice:

I chose "4" because of each support term of the PHP versions.

Then these directories/files were made:

$ ls /etc/php*
/etc/php-7.2.ini          /etc/php-fpm.conf

/etc/php-7.2:
server
/etc/php-7.2.sample:
gd.ini       opcache.ini

Well, the .ini files in /etc/php-7.2.sample are PHP extensions.
According to necessity, copy each of them to /etc/php-7.2/ to activate the extensions:

# ln -sf /etc/php-7.2.sample/%target-ini-files% /etc/php-7.2/

Also edit /etc/php-7.2.ini as needed.
For example:

; Maximum allowed size for uploaded files.
; http://php.net/upload-max-filesize
;upload_max_filesize = 2M
upload_max_filesize = 5M

Besides, the manual is also installed as /usr/local/share/doc/pkg-readmes/php-7.2 which declares:

The main OpenBSD php packages include php-fpm, FastCGI Process Manager.
This manages pools of FastCGI processes: starts/restarts them and maintains a minimum and maximum number of spare processes as configured. You can use rcctl(8) to enable php-fpm at boot, and start it at runtime:2

rcctl enable php72_fpm
rcctl start php72_fpm

OK. We're ready.
Let's start daemon:

# rcctl enable php72_fpm
# rcctl start php72_fpm

* Troubleshooting: If you fail to start php72_fpm (or php71_fpm), it might be the known bug.

Usage

Next, we have to prepare a web server.
So let's edit /etc/httpd.conf to add fastcgi socket in SERVERS sections like this:

types {
    include "/usr/share/misc/mime.types"
}

ext_addr="egress"

server "default" {
    listen on $ext_addr port 80

    root "/htdocs"
    directory index index.php

    location "*.php*" {
        fastcgi socket "/run/php-fpm.sock"
    }
}

Note that chroot works in this context.
Therefore, fastcgi socket "/run/php-fpm.sock" in /etc/httpd.conf actually means fastcgi socket "/var/www/run/php-fpm.sock".
This is the same to that root "/htdocs" means "/var/www/htdocs".

Testing

Let's make /var/www/htdocs/index.php for testing like this:

# echo "<?php phpinfo(); ?>" > /var/www/htdocs/index.php
# # delete the file above afterwards as needed

Then browsing your host will show you the whole phpinfo!