Make sure your PHP setup has PDO installed (it is enabled by default in PHP 5.1.0 or above), and the PostgreSQL PDO driver is enabled. Then a database connection can be established with:
<?php
$dbh = new PDO('pgsql:host=localhost;dbname=[YOUR_DATABASE_NAME]');
?>
The default PHP that comes with OS X 10.9 does not have Postgres support. The easiest way to use PHP with Postgres is to use MAMP.