In order to help out, CodeIgniter uses a series of function, method, class and variable names in its operation. Because of this, some names cannot be used by a developer. Following is a list of reserved names that cannot be used.
Since your controller classes will extend the main application controller you must be careful not to name your methods identically to the ones used by that class, otherwise your local methods will override them. The following is a list of reserved names. Do not name your controller any of these:
is_php()is_really_writable()load_class()is_loaded()get_config()config_item()show_error()show_404()log_message()set_status_header()get_mimes()html_escape()remove_invisible_characters()is_https()function_usable()get_instance()_error_handler()_exception_handler()_stringify_attributes()$config$db$lang
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https://www.codeigniter.com/user_guide/general/reserved_names.html