codeigniter自定义库的常用函数

I need to create custom libraries for my Codeigniter 3 application that have common functions used by all my custom library.

How to use function only in one place?

You can create new custom libraries. The library classes created by you should be placed in your application/libraries folder.

Classes should have basic prototype like this (here the name Myclass as an example):

 <?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Myclass {

    public function my_function()
    {
    }
}

/* End of file Myclass.php */

From within any of your Controller functions you can initialize your class using the standard:

$this->load->library('myclass');

$this->myclass->my_function(); // Object instances will always be lower case

What you appear to need is a helper file, not a library. Create a file in application/helpers directory

Eg file : mycommon_helper.php Contents as :

<?php if(!defined('BASEPATH')) exit('No direct script access allowed');

function my_function($param1) {
   printf('Hello %s', $param1);
}

Include this in the autoload file as below

$autoload['helper'] = array('url','html', .....'mycommon');  // Note _helper is omitted

After this you use the my_function($param1) anywhere in your code

 $sayIt = my_function('user2473015');