How to set up forum to email validation email to new applicants?

[amylase]

Hi

I have a discussion forum (using Discuz!7.01) running on Ubuntu 8.10 with PHP5, MySQL and Apache2. A problem I have now is unable to send emails to new users' emailboxes. When new people register to join this forum, I'd like the forum to email some sort of validation instructions to the applicant.

In the settings of Discuz!7.01 I can choose to set it up to email
"through PHP function" or "through SMTP server to send SOCKET (ESMTP authentication support)".

How do I actually set up either of them? (whichever is quicker and simpler). All I need is for the forum to send email to new applicants when they register for the first time.

Currently I ticked to go "through PHP function". There is also a "test email" button. I pressed the button and see a pop up window saying "success". But when I go check mailbox, there's no email sent from the forum. I guess that means I didn't set up PHP to handle emailing properly. I'm a bit lost about what to do from here. Please help...

Thanks a lot.