Inspired by @corum in this thread I tested LAM 6.7 and a newer PHP version (>=5.6) is needed so I used nethserver-rh-php71-php-pfm.
At step 2 of the howto I additionally did the following:
yum -y install nethserver-rh-php71-php-fpm rh-php71-php-ldap
Add the FilesMatch/SetHandler to /etc/httpd/conf.d/lam.apache.conf at line 9 so it looks like this:
<Directory /usr/share/ldap-account-manager>
  Options +FollowSymLinks
  AllowOverride All
  Require all granted
  DirectoryIndex index.html
  <FilesMatch \.php$>
      SetHandler "proxy:fcgi://127.0.0.1:9001"
  </FilesMatch>
</Directory>
Restart relevant services.
systemctl restart httpd rh-php71-php-fpm