Out-of-the-box WordPress assumes it’s going to be talking to a MySQL database using PHP’s original mysql extension instead of the newer mysqli extension (true up to WordPress 2.1.3 at least). If for some reason you need to run using mysqli, there are simple instructions on the WordPress support site: MySQLi for WordPress 2.1.x.

Switching to MySQLi was easy and worked as described, except for one thing: I couldn’t upgrade WordPress. After a bit of sleuthing I found there’s a MySQL version check buried in the upgrade process. For the current version (WordPress 2.1.3) it’s checking that MySQL is version 4.0.0 or higher. According to the PHP documentation for mysqli, it needs to run on MySQL 4.1 or above - so the version check is redundant.

Short-term fix: follow the first few steps of the WordPress upgrade process but before running the upgrade program edit wp-admin/upgrade-functions.php and gut this function:

function wp_check_mysql_version() {
  // (version-check removed)
}

You should be able to run the upgrade program fine now.