[Brandon Martus]

Joel suggested the correct PHP function.

To do it in PERL, you could do this:

  $someText =~ s/\n\n/<p>/g;
  $someText =~ s/\n/<br>/g;

And, to answer your question about the spell checker.

vBulletin has it commented out by default, because I believe the service they were using started to charge people.
They just leave the code there in case you want to pay for the service, I think.