Codeigniter initialize() function not working
$this->load->initialize(); loads some libraries at the very parent class, dbdriver is 1 of that packages so if you are using php7.2, 7.3, 7.4 change line
$db['default']['dbdriver'] = 'mysql';
$db['default']['dbdriver'] = 'mysqli';
in your codeigniter database.php file.
mysql functions become depracated in php >= 7, you should use msqli (i - improved)