Use and precautions of set_include_path and get_include_path

set_include_path sets the default include path
For include or require files, look in the default include path and then in the same directory
Ex. :
D: / xampp/htdocs/test/under include 1 file for cls. mysqli. php
D: under/xampp htdocs test / 1 file for set_include_path. php
The content inside is

 
set_include_path('/xampp/htdocs/test/include');　　// Please note that php The inside of the '/' Does not represent the root of the site , It's the disk character where the file is located , available echo realpath('/'); To view  
// if php Version too low, this function is not supported , Available general edition ini_set() 
/* View the true default include path */ 
echo realpath(get_include_path()); 
/* To include cls.mysqli.php Only need to */ 
require_once('cls.mysqli.php'); 
$mysql = new Mysql('localhost','root','','test');　　// There are no errors in the instantiation and subsequent query statements