How to implement PDO-based preprocessing in PHP

$servername="localhost";
$username="root";
$password="admin";
//$dbname为我的test数据库
$dbname="test";
try{
  $conn=new PDO("mysql:host=$servername;dbname=$dbname",$username,$password);
  //设置pdo错误异常处理
  $conn->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
  //预处理SQL并绑定参数
  $stmt=$conn->prepare("INSERT INTO user(user_first,user_last,age)VALUES(:user_first,:user_last,:age)");
  $stmt->bindParam(":user_first",$user_first);
  $stmt->bindParam(":user_last", $user_last);
  $stmt->bindParam(":age",$age);
  //要执行的插入记录
  $user_first="xiao";
  $user_last="liang";
  $age=15;
  $stmt->execute();
  //要执行的插入记录
  $user_first="xiao";
  $user_last="cheng";
  $age=23;
  $stmt->execute();
  echo "News record created successfully!";
}catch(PDOException $e){
  echo "Error:".$e->getMessage();
}
$conn=null;