PHP PDO->exec() executes SQL statements without returning a result set_PHP tutorial

<?php
/*
DO->exec() 方法主要是针对没有结果集合返回的操作，
比如 INSERT、UPDATE、DELETE 等操作，它返回的结
果是当前操作影响的列数
 
语法：PDO->exec( string statement )
*/
    //构造PDO连接
    header("Content-type:text/html;charset=utf-8");
    $dbh = "mysql:host=localhost;dbname=test";
    $db = new PDO($dbh, &#39;root&#39;, &#39;123456&#39;);
    $db->query("set character set &#39;utf8&#39;");
    //写入数据
    $username="liming";
    $password = md5("123456");   
    $regdate = time();//返回的是一个整形，所以下面语句中可以不带引号
    $sql_exec = "INSERT INTO userlist (username,password,regdate)VALUES(&#39;$username&#39;,&#39;$password&#39;,
    $regdate)";
    $count = $db->exec($sql_exec);
    echo &#39;写入 &#39;.$count.&#39; 条数据记录！&#39;;
 
    echo "<hr/>";
    $sql_select = "SELECT * FROM userlist";
    $sth = $db->query($sql_select);//$sth 是结果集对象
    //$sth->setFetchMode(PDO::FETCH_ASSOC);// 如果不在setFetchMode()中指定返回的结果类型，也可以单独使用fetch()方法设定
    while($row = $sth->fetch(PDO::FETCH_ASSOC)){
    print_r($row);
    echo "用户名:".$row[&#39;username&#39;]."  ";
    echo "密码:".$row[&#39;password&#39;]."  ";
    echo "注册时间:".date("Y-m-d",$row[&#39;regdate&#39;])." &nbsp";
    }
 
?>