lost connection from apache to postgres: "can't resolve symbol PQsetdb"

Hello, all.

Somehow our web server (which is Apache 1.3.0 running PHP 3.0.5) has
forgotten how to talk to Postgres.

So far I don't have the slightest idea what went wrong. The only
significant changes to the environment are that we've been doing a bit of
painful development with mod_perl and recently upgraded to 6.4 (although
the problem didn't appear until at least a couple weeks after the Postgres
upgrade).

In any case, the only error message I've found so far is from the web
server's error_log:

/web/bin/httpd: can't resolve symbol 'PQsetdb'

I've searched through the mailing lists and can't find much about this. If
anyone could give me any pointers as to how to even *diagnose* the problem,
I'd be very grateful. Sorry I don't have more info to provide -- I just
don't know where to start looking.

Thanks in advance.

Charlie