If you are fetching issue with undefined function site_url() in your codeigniter 3 application then you have to load "url" helper on auto load.
I wanted to share this error, it will comes when you install fresh codeigniter 3 application and try to run, you will find this error undefined function site_url() when you use site_url() helper.
So, if you used site_url() helper then you need to load "url" helper, that will provide site_url(), bese_url() etc helper function. So you have to simple load "url" helper like as following.
$autoload['helper'] = array('url');
After above line add, you will solved your issue defiantly.
I hope you found your best solution.. :) :)