php使用pdo连接mysql如何是指长连接？

只要设置$db->setAttribute (\PDO::ATTR_PERSISTENT, true);就好了，代码如下：

<?php
$outhtml = "";
try {
    $db = new PDO('mysql:host=localhost;dbname=guest', 'guest', 'p6yhG8iDRMNxdJ9w');

    $db->setAttribute (\PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION);
    $db->setAttribute (\PDO::ATTR_PERSISTENT, true);
    $res = $db->prepare("select * from User"); //准备查询语句
    $res->execute();
    while ($row = $res->fetch(PDO::FETCH_ASSOC)) {
        if (empty($key)) {
            $key = array_keys($row);

            $outhtml .= "<tr>";
            foreach ($key as $item) {
                $outhtml .= "<th>{$item}</th>";
            }
            $outhtml .= "</tr>";
        }
        $values = array_values($row);
        $outhtml .= "<tr>";
        foreach ($values as $item) {
            $outhtml .= "<td>{$item}</td>";
        }
        $outhtml .= "</tr>";
    }


} catch (\PDOException $e) {

    $outhtml .= $e->getMessage();

}
echo $outhtml."\n";
$db = null; //关闭数据库