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