problem when trying to install PHPO with postgresql support

When trying to build with PostgreSQL support I always get the following
message : Cannot find libpq-fe.h. I reinstalled PostgreSQL , but am
unable to find this file . Does anyyone have a clue where this file can
be obtained or what I can do ?
Postgresql = 7.3
PHP = 4.2.3

Thanx in advance for your cooperation.

Wim