if ($INSTALL['dbtype'] == 'mysql') { /// Check MySQL extension is present
if (!extension_loaded('mysql')) {
$errormsg = get_string('mysqlextensionisnotpresentinphp', 'install');
$nextstage = 3;
}
}
The config.php being missing shouldn't cause a blank screen. Both index.php and admin/index.php will redirect to install.php if config.php isn't found.