I'd bet the version of PHP you're using doesn't have the PostgreSQL libraries installed in the build...
If you have the ability, recompile the PHP interpreter with the '--with-pgsql=/path/to/PostgreSQL' option during the configure stage.
If the PHP interpreter resides on a remote machine that you don't have access to, see if the web-meister can build you a new PHP module; if you built the module from the sources, make sure this option is set when you compiled the interpreter.
If you downloaded a pre-compiled binary version (say you don't have a compiler, or whatever), remember that support for PostgreSQL is not enabled by default. You can either find a PHP binary with PostgreSQL support built in, or build your own interpreter from the source code.