PHP实现链式操作的原理详解

<?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();