The SPL extension appears to be the one which provides the DirectoryIterator class, which is used in several places in Moodle 2.0 (was not used in 1.9).
This extension is not included on the environment.php list of required extensions. Should it be?
Note: This extension is part of default, standard installations of php. However, it is not a required part of the core system until 5.3 (Moodle only requires 5.2.8), according to http://cz.php.net/manual/en/spl.installation.php
I came upon this error because the extension does not seem to be provided by the Quercus php-in-java implementation. Sigh.