$ This-> config, $ this> input call in the php-CI framework

In the company's project program, the custom class directly uses $ this-& gt; config-& gt; item () to reference the methods in the system definition class congfig, I would like to ask all the great gods. $ This-& gt; config-& gt; item () at this time, config serves as the attribute of the current class, but no matching $ this-& gt; is found at all; config. The system file Codeigniter. php contains the following statement: $ CFG & amp; load_class (& #39; Config & #39;, & #39; core & #39 ;). But this is in the custom Framework php

In the company's project program, the custom class directly uses $ this-> config-> item () to reference the methods in the system definition class congfig. I would like to ask various great gods. $ This-> config-> item () is the attribute of the current class, but no definition of $ this-> config is found for the current class.
The system file Codeigniter. php contains the following statement: $ CFG = & load_class ('config', 'core '). However, this is where the Config. php file is loaded outside the custom class. Why can I directly use $ this-> config-> item () in the current class (). Hope you can help me solve this problem.