PHP mail and DKIM


I activatet dkim-milter as in this doc :

In the log i see:
Jul 14 15:08:35 s3 dkim-filter[29051]: Sendmail DKIM Filter v2.8.3 starting (args: -x /etc/dkim-filter.conf -u dkim-filter -P /var/run/dkim-filter/ -p inet:8891@localhost -b s)

So i assume things are working. But when i send a mail with the linux mail command, i do not see the DKIM headers in the mail, neither when i send it with PHP. How can i achieve that the mails which are being send by PHP are getting signed?

Hmm, i enabled DKIM on three Virtualmin servers now, and only one seems to work. When i use the linux mail-function on this server, it sends a DKIM signed mail, on the other 2 servers the mails do not get signed. Anybody?