Untar the tar.gz archive into
php4/ext (Latest official releases
can be found at SourceForge
» PHP Form Extension Project)
If the new directory is now called something like
session_pgsql. You should name it to
session_pgsql (except you only want to build it as
self-contained php-module).
Run ./buildconf
in php4
Run configure--with-session-pgsql (and your other
options)