last_query
is the raw SQL. PHP ActiveRecord is using PDO's prepared statement so you won't see the actual query.
What you should do is using a logger. The values will be shown there (the code).
$log = Log::singleton('file', 'my.log'); //using PEAR Log
ActiveRecord\Config::instance()->set_logging(true);
ActiveRecord\Config::instance()->set_logger($log);
Example from: http://www.phpactiverecord.org/boards/4/topics/1119-re-show-last-sql-query