WordPress Won't Connect to DB, But Test Code Does

I closed my last question because I thought my problem wasn’t WordPress related. After chasing some ghosts, I’m back to WordPress as the culprit. I’m getting the infamous “Error establishing a database connection” message when I invoke index.php on a brand new WordPress install. This is relevant information from my wp-config:

define(‘DB_NAME’, ‘gazos’);
define(‘DB_USER’, ‘root’);
define(‘DB_PASSWORD’, ‘somepassword’);
define(‘DB_HOST’, ‘localhost:3306’);

I also tried localhost without the port and got the same error.

To isolate the problem, I ran the following code from the same directory as WordPress via the browser:

<?php
$mysqli = new mysqli('localhost:3306', 'root', 'somepassword', 'gazos');

if ($mysqli->connect_error)
    die('Connect Error (' . $mysqli->connect_errno . ') '. $mysqli->connect_error);
else
    echo 'Good connection to gazos';
?>

I get a good connection. It fails if I remove the port. I’m running PHP 5.4.13 and MySQL 5.0.10. Could it be that my PHP version is too new?