php如何利用pdo方式关闭数据库

<meta http-equiv="content-type" content="text/html" charset="utf-8"/>
<form method="POST" action="<?php echo iconv("GB2312","UTF-8",htmlspecialchars($_SERVER['PHP_SELF']));?>">
服务器地址：<input type="text" name="mysqlPDOip"/><br/>
服务器账号：<input type="text" name="mysqlPDOuser"/><br/>
服务器密码：<input type="password" name="mysqlPDOpassword"/><br/>
<input type="submit" value="连接mysql"/>
</form>
<?php
if($_SERVER['REQUEST_METHOD']=="POST"){//if判断是否POST提交
    $servername=$_POST['mysqlPDOip'];
    $username=$_POST['mysqlPDOuser'];
    $password=$_POST['mysqlPDOpassword'];
    try{
        //$conn = new PDO("mysql:host=$servername;dbname=myDB",$username,$password);
        $conn = new PDO("mysql:host=$servername;dbname=mysql", $username, $password);//连接数据库
        echo "连接成功！";
        echo "<br/>你所连接的服务器地址：".$servername;
        echo "<br/>你所连接的服务器账号：".$username;
        echo "<br/>你所链接的服务器密码：".$password;
        $conn = null;//关闭数据库连接
        echo "<br/>已成功关闭数据库";
    }
    catch(PDOException $e)
    {
        echo $e->getMessage();
    }
}
?>

try
{ //...}
catch(Exception $e)
{ //...}