Can't Get PHP Going on PostgreSQL

I am new to both PHP and PostgreSQL. Trying to install PHP and PostgreSQL 8.0 on Slackware Linux 10.0.0 . 
I am getting the following error out of Apache:
   "Fatal Error: Call to undefined function: pg_connect() ...."

PostgreSQL is up and running, and appears to be fine. 

Have found a lot of web references to this; the best was http://us2.php.net/pgsql . But am unable
to proceed because I am not sure what to do. 
 
Facts about what I have installed: 
- Installed PHP from Slackware binary; I made no special efforts to compile it. According to a PHP utility
  function (phpinfo()), looks like it might be configured for MySQL already. 
- version info:
    PHP =  4.3.7
    Linux kernel = 2.4.26
   Apache = 1.3.31
 
How do I find these functions and install OR configure the php.ini file?
 
I don't think this is hard to fix, but it is beating me up right now. What I need to find a blow-by-blow of 
what to do. Can somebody help me?