I have a controller welcome.php and a method cat(). This is the query string http://localhost/CodeIgniterhi/?welcome/cat/0. When I click on that link I can't access the method cat() in controller welcome.php. Why? Is there any routing problem? My default router is welcome/index.
Assuming you removed index.php from your urls, the problem is that you have query char in url
/CodeIgniterhi/?welcome/cat/0
should be
/CodeIgniterhi/welcome/cat/0
if you have problem that you can't remove the query char ? you should try enabling in config/config.php this:
$config['uri_protocol'] = 'QUERY_STRING';
sincerily is not the best practice to use query strings in url, they will look not SEO friendly