I don't know how you've got things set up, but it's possible
you have compiled the pgsql support as a loadable module,
in which case you will need to load it.
There are several ways to do this. If your site uses the pg_*
stuff extensively then you probably want to put
extension=pgsql.so
in your php.ini script. Otherwise, you just need to stick something like <?php dl(pgsql.so); ?> in your page.