Thanks for all the advice, some of you said that in one of my $_POST
variables I was missing a single quote. That was simply a mistake
typing my message into google groups.

The link about escape sequences and strings was very ful with
another problem I was having. But I found the problem with my form in
the meantime. In another php file I had some code to set a
$_SESSION['blahname'] variable written in the wrong place. It was off
by one brace, and didn't throw any errors. I put it in the right place
and the query worked fine now that I wasn't trying to insert a NULL
value into a column that was constrained not to take NULL's.

So, my problem was actually related to SQL, though none of you could
probably tell. My advice to anyone who stumbles upon this thread is to
double check their $_SESSION variables. In my case my syntax was
correct, but a one line logic error on line 143 foiled my SQL query in
a completely different place. I'm a little embarrassed to say it took
me hours to spot the mistake it was so buried in there.