After upgrading to PHP 4.2.3 from PHP 4.1.2 (Red Hat Linux Advanced Server with Stronghold 4.0) in order to manually compile in MHASH support, I discovered that Postgres support has disappeared, despite being specified on the command line, and compiling with no errors.
FATAL: Undefined function: pg_connect()
Confirmed by looking at the output of phpinfo() and comparing it to the output pre-upgrade - no mention of PostgreSQL in the new one.
Detective work led me to find that the old pgsql.so in /usr/lib/php4 was untouched, and the new one had ended up in /usr/lib/20020429 instead.
The fix was to edit config_vars.mk after configuration to change the value of EXTENSION_DIR, and then compile.
Not quite sure where 20020429 came from, looks like a left-over value from development testing...
Anyway, in case any one else has a similar problem, thought I'd document it here, as a problem with pg_connect is where this will first surface as a symptom.