To install DokuWiki using the OpenBSD packages use the package manager:

~$ doas pkg_add dokuwiki

The package managers will take care of setting up dependencies and chroot requirements.

You may want to change your httpd.conf to something similar show below.

server "default" { 
        listen on egress port 80 
        listen on 127.0.0.1 port 80 
 
        location "/dokuwiki/*.php*" { 
                root { "/dokuwiki", strip 1 } 
                fastcgi socket "/run/php-fpm.sock" 
        } 
        location "/dokuwiki/*" { 
                directory index index.php 
                root { "/dokuwiki", strip 1 } 
        } 
}

Alternative configuration with updated syntax (tested on OpenBSD 6.6):

server "default" {
  listen on egress port 443
  root "/dokuwiki"
  directory index doku.php
 
  # Set according to upload_max_filesize and post_max_size in php.ini
  connection max request body 20971520
 
  location "*.php"     { fastcgi socket "/run/php-fpm.sock" }
  location "/data/*"   { block }
  location "/conf/*"   { block }
  location "/bin/*"    { block }
  location "/inc/*"    { block }
  location "/vendor/*" { block }
}