PDO预处理对象参数绑定后,执行查询的结果竟然不是数据表的字段值，而成了字段名，请教一下，这是为什么？

Question

//使用PDO对象连接数据库 try{ $pdo = new PDO("mysql:dbname=origin;host=localhost","root",""); } catch (PDOException $e){ die("数据库连接失败：".$e -> getMessage()); } //使用占位符准备SQL语句 $sql = "select ? , ? , ? from user where age = ? "; //进行预处理 $stmt = $pdo -> prepare($sql); //绑定参数 $stmt ->bindparam(1,$id); $stmt ->bindparam(2,$name); $stmt ->bindparam(3,$age); $stmt ->bindparam(4,$num); //给参数赋值 $id ='id'; $name = 'name'; $age ='age'; $num = 100; //执行 $stmt -> execute(); //处理结果集 while ($result = $stmt->fetch()){ var_dump($result); } /*程序运行结果: Array ( [id] => id [name] => name [age] => age ) Array ( [id] => id [name] => name [age] => age ) Array ( [id] => id [name] => name [age] => age ) Array ( [id] => id [name] => name [age] => age ) Array ( [id] => id [name] => name [age] => age ) Array ( [id] => id [name] => name [age] => age ) Array ( [id] => id [name] => name [age] => age ) Array ( [id] => id [name] => name [age] => age ) 结果竟然全部是字段名，而不是字段值，不知道是为什么？ */

Answer 1

我把sql写出来, 你就明白了.

select "id", "name", "age" from user where age = "100"