PHP数据库扩展mysql、mysqli及pdo

<?php

  $conn = mysql_connect("localhost", "root", "") or die("Mysql connect error");

  mysql_select_db("test", $conn);

  $res = mysql_query("select * from user");

  var_dump($res);

?>

<?php

  $conn = mysqli_connect("127.0.0.1", "root", "", "test") or die("Mysql connect error".mysqli_connect_error());

  $res = mysqli_query($conn, "select * from user");

  while ($row = $res->fetch_object()) {

    var_dump($row);

  }

?>

<?php

  $pdo = new pdo("mysql:host=127.0.0.1;dbname=test", "root", "");

  $query="select * from user";//需要执行的sql语句

  $res = $pdo->prepare($query);//准备查询语句

  $res->execute();

  while($result = $res->fetch(PDO::FETCH_ASSOC)) {

    var_dump($result);

  }

?>