Sending mass mail with PHP

Hi! I've been reading the previous postings on the subject and started wondering if it was possible to create sendmail queue files directly with PHP. What I mean is that I would have to get the recipients from the db, write the needed queue files and then fire sendmail up commanding it to handle the queue just created. Comments of this would be very welcome because this way I would have to open only one sendmail process per posting and this problem of having to send many (perhaps over 100k) emails would otherwise probably kill the server. Oh, and btw, I'm not trying to build any spamming machine, the users have ordered these messages. - markus