The principle of PHP class automatic loading and PHP chain operation

<?php
namespace Tests;
class Test1{
  static function test(){
    echo __CLASS__.&#39;<br>&#39;;
    echo __FILE__.&#39;<br>&#39;;
  }
}

<?php
include "load.php";
Tests\Test1::test();

<?php
class Loader
{
  static function loadClass($class)
  {
    $class =  __DIR__.DIRECTORY_SEPARATOR.str_replace(&#39;\\&#39;,&#39;/&#39;,$class).&#39;.php&#39;;
    if (file_exists($class)) {
      include $class;
      return;
    }
  }
}
spl_autoload_register(array(&#39;Loader&#39;,&#39;loadClass&#39;));

<?php
 
class db
{
    public function where()
    {
    //code here
    }
    public function order()
    {
    //code here
    }
    public function limit()
    {
    //code here
    }
}

<?php
 
$db = new db();
 
$db->where();
$db->order();
$db->limit();

<?php
 
class db
{
    public function where()
    {
    //code here
    return $this;
    }
    public function order()
    {
    //code here
    return $this;
    }
    public function limit()
    {
    //code here
    return $this;
    }
}

<?php
 
$db = new db();
 
$db->where()->order()->limit();