当PHP 使用PDO访问数据库且脚本需要长时间执行时，频繁会遇见’ MySQL server has gone away’的错误。分析问题产生原因：因为脚本较长时间未与数据库进行通信，导致数据库连接超时与服务器断开连接导致，这时使用断开的数据库连接操作数据库（CRUD），就会产生’ MySQLserver has gone away’的错误提示。解决之道在于长时间运行的脚本需要与服务器保持心跳，一旦检测到连接断开，则需要重新连接数据库。

在考虑保持最小数据库负载（检测手段开销小，心跳频率不能太过频繁）和较高脚本健壮性（需要更高频率的心跳），可以选择不同的心跳测试策略和心跳频率。以下的代码给出了一个解决办法，通过单件封装了数据库连接，当每次获取数据库连接时会判断是否需要检测连接仍然有效，如果连接失效则重建连接，在脚本健壮性与服务器压力之间保持一个最好的平衡点

<?php
/*

• 使用单件来维护数据库连接，当需要数据连接时需要从单件获取，这样在单件内部可以检测并恢复数据连接

*/

class DBConnection
{

const RECHECK_FREQUENCY = 300;// 5 minutes
private static $instance = null;
private $dbh = NULL;
private $lastCheckTime = 0;

private function __construct()
{
    $this->lastCheckTime  =time();
}

public static function instance()
{

    if (NULL ==self ::$instance)
    {
        self:: $instance =newDBConnection();
    }
    return self::$instance;
}

public function dbh()
{
   
    $this->ensureConnection2();
   
    return $this->dbh;
}

private function ensureConnection()
{
    if (is_null($this->dbh))
        return $this->makeConnection();
   
    try
    {
        $status = $this->dbh->getAttribute(PDO::ATTR_SERVER_INFO);
        error_log('MySQL server checked been there');
    }
    catch(PDOException$e)
    {
        if((int)$e->errorInfo[1]== 2006 &&$e->errorInfo[2] == 'MySQLserver has gone away')
        {
            error_log("MySQLserver has gone away, try to reconnection...");
            return $this->makeConnection();
        }
       
        error_log('Get db server attribute failed: ' .$e->getMessage());
    }
   
    return $this->dbh;
}

private function ensureConnection2()
{
    if (is_null($this->dbh))
        return $this->makeConnection();
   
    try
    {
        $now = time();
       
        if ($now -$this->lastCheckTime> self::RECHECK_FREQUENCY)
        {
            $this->lastCheckTime  =$now;
            $status = $this->dbh->query("select1");
            error_log('MySQL server checked been there');
        }
       
    }
    catch(PDOException$e)
    {
        if((int)$e->errorInfo[1]== 2006 &&$e->errorInfo[2] == 'MySQLserver has gone away')
        {
            error_log("MySQL server has gone away, try to reconnection...");
            return $this->makeConnection();
        }
       
        error_log('Get db server attribute failed: ' .$e->getMessage());
    }
   
    return $this->dbh;
}

private function makeConnection()
{
    try
    {
        $options = array (
            PDO:: MYSQL_ATTR_INIT_COMMAND => "set names 'utf8'",
            PDO:: ATTR_PERSISTENT => false,
            PDO:: ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
        );
        $this->dbh = new PDO(DB_DSN, DB_USER, DB_PASSWORD, $options);
        return $this->dbh;
    }
    catch (PDOException$e)
    {
        error_log('Connection failed: ' .$e->getMessage());
        exit();
    }
   
    return null;
}

}