使用的重要性是什么:$stmt->free_result();$stmt->close();在使用准备好的语句调用数据库之后:$mysqli=newmysqli("database","db","pass","user");$stmt=$mysqli->prepare("SELECTemailFROMusersWHEREid=?");$stmt->bind_param('i',$_SESSION['id']);$stmt->execute();$stmt->bind_result($email);while($stmt->fetch()){echo$email;}$stmt->free