Yum Update Errors

I have been running Postgres 8.3 on CentOS for abut 6 months and using yum to update it. Recently, I started getting error messages saying that I'm missing libpq.so.3 which is needed by packages perl-DBD-Pg, mod_auth_pgsql and php-pgsql. But the file IS there, in /usr/lib. Libpq.so.5 is also there, if that has any bearing.

I have not recently installed any new RPM packages nor programs. I have postgresql 8.3.7-1PGDG, perl-DBD-Pg 1.31-6 and php-pgsql 4.3.9-3.22.15.

I seem to recall having a similar problem before but I can't remember anything about how to solve it.

Thanks in advance,
Bruce