PHP implements the principle of chain operation

<?php

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

$db = new db();

$db->where();
$db->order();
$db->limit();如果要实现链式调用，这要在方法的结束添加return $this即可。
db.php
<?php

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

$db = new db();

$db->where()->order()->limit();