Well ..... being curious about this, I decided to spend 10 minutes on the problem on getting back from the Windoze wrestling match.
I 'ran' phpinfo and checked that pgsql was visible. You should see
'--with-pgsql=shared'
amongst the configure command output at the top of the page. You should also find a whole 'pgsql' section later on in the output. If either of these is missing you'll need to figure out why. Check back to the source of your PHP installation to make sure that '--with-pgsql' was specified. Make sure too that you've actually restarted the httpd service!
(# service httpd restart)
Anyway, in my case both were present.
I then made an ibay for phpPgAdmin, set all the usual bits to allow execution of dynamic content, downloaded the phpPgAdmin tarball, expanded it into the ../html part of the ibay, edited ..../conf/config.inc.php to allow logging onto the database with the username 'postgres' (just one edit, changing true to false!!), and then visited the ibay with a web browser. It all worked first time for me!!
I think you need to check your PHP installation!!
Anyway, on this occasion I got lucky and so I'm off for a

and a curry!
Regards,
Dave