ThinkPHP などのさまざまなフレームワークにおける SQL 操作チェーン記述の原理の簡単な分析

<?php
class Test{  
  private $var = "";
  public function Func(){
        $this->var = "Var is change";
        return $this;
    }}    
    $obj = new Test();
    var_dump($obj);

 var_dump($obj->Func());

object(Test)[1]

 private 'var' => string ''

(length=0)object(Test)[1]

private 'var' => string 'Var is change'

 (length=13)

<?php   
 class UserModel{
        private $field     ="*"; 
        private $tableName ="";
        private $where     ="";
        private $order     ="";
        private $limit     ="";
        function field($field){
            $this->field = $field;
            return $this;
        } 
       function table($tableName){
            $this->table = $tableName;
            return $this;
        } 
       function order($order){
            $this->order = "ORDER BY ".$order;
            return $this;
        }
        function where($where){
            $this->where = "WHERE ".$where;
            return $this;
        }
        function limit($index, $limit = 0){ 
           $this->limit = "LIMIT ".$index; 
           if($limit){ 
               $this->limit.= ",{$limit}";
            }
            return $this;
        }        
       function select(){
            if(empty($this->tableName)){
                $this->tableName = str_replace("Model", "", __CLASS__);//如果表名不指定，则获取类名
            }
            $selectSql ="SELECT {$this->field} 
                FROM `{$this->tableName}` 
                         {$this->where} 
                         {$this->order} 
                         {$this->limit}"
                         FROM `{$this->tableName}` 
                         {$this->where} 
                         {$this->order} 
                         {$this->limit}";
             //构造SQL语句模版串
            echo $selectSql;
            //return mysql_query($selectSql);  执行拼接后的SQL语句
        }
    }    
     $user = new UserModel();
     $user->where("`user` = 1")->order("`user` DESC")->limit(5)->select();
?>