Add the SPL extension to the list of requirements in the web interface.

This is needed for the autoloader to work, or code calling class_implements(), among others.

modules/core/www/frontpage_config.php

@@ -106,6 +106,7 @@ $functionchecks = array(
 	'dom_import_simplexml' => array('required', 'XML DOM'),
 	'preg_match'       => array('required',  'RegEx support'),
 	'json_decode'      => array('required', 'JSON support'),
+	'class_implements' => array('required', 'Standard PHP Library (SPL)'),
 	'curl_init'        => array('optional', 'cURL (required if automatic version checks are used, also by some modules.'),
 	'mcrypt_module_open'=> array('optional',  'MCrypt (required if digital signatures or encryption are used)'),
 	'session_start'  => array('optional', 'Session Extension (required if PHP sessions are used)'),