CodeIgniter连贯操作的底层原理分析

<?php
class test
{
 public $a='';
 public $b='';
 public function actiona() {
  $this->a="hello";
  return $this;
 }
 public function actionb() {
  $this->b="world";
  return $this;
 }
 public function actionc() {
  echo $this->a." ".$this->b;
 }
}
$oktest=new test();
$oktest->actiona();
$oktest->actionb();
$oktest->actionc();
?>

<?php
class test
{
 public $a='';
 public $b='';
 public function actiona() {
  $this->a="hello";
  return $this;
 }
 public function actionb() {
  $this->b="world";
  return $this;
 }
 public function actionc() {
  echo $this->a." ".$this->b;
 }
}
$oktest=new test();
$oktest->actiona()->actionb()->actionc();
?>

return $this

<?php
class sql{
 public $select;
 public $from;
 public $where;
 public $order;
 public $limit;
 public function from($_from='FROM test') {
 $this->from=$_from;
 return $this;
 }
 public function where($_where='WHERE 1=1') {
 $this->where=$_where;
 return $this;
 }
 public function order($_order='ORDER BY id DESC') {
 $this->order=$_order;
 return $this;
 }
 public function limit($_limit='LIMIT 0,30') {
 $this->limit=$_limit;
 return $this;
 }
 public function select($_select='SELECT ') {
 $this->select=$_select;
 return $this->select." ".$this->from." ".$this->where." ".$this->order." ".$this->limit;
 }
}
$sql =new sql();
echo $sql->from()->where()->order()->limit()->select();
?>