<?php
// find 'path' in table 'core_config_data' e.g. 'design/head/demonotice'
$my_change_config = new Mage_Core_Model_Config();
// turns notice on
$my_change_config->saveConfig('design/head/demonotice', "1", 'default', 0);
// turns notice off
$my_change_config->saveConfig('design/head/demonotice', "0", 'default', 0);
?>