pcntl_fork后提示MySQL server has gone away

<?php
[/font][/backcolor][/color][color=#ff800]// Create the MySQL connection
[/font][/backcolor][/color][color=#00bb]$db = mysql_connect($server, $username, $password);

[/font][/backcolor][/color][color=#00bb]$pid = pcntl_fork();

if ( [/font][/backcolor][/color][color=#00bb]$pid == -1 ) {
[/font][/backcolor][/color][color=#ff800]// Fork failed
[/font][/backcolor][/color][color=#0770]exit(1);
} else if ( [/font][/backcolor][/color][color=#00bb]$pid ) {
[/font][/backcolor][/color][color=#ff800]// We are the parent
// Can no longer use $db because it will be closed by the child
// Instead, make a new MySQL connection for ourselves to work with
[/font][/backcolor][/color][color=#00bb]$db = mysql_connect($server, $username, $password, true);
} else {
[/font][/backcolor][/color][color=#ff800]// We are the child
// Do something with the inherited connection here
// It will get closed upon exit
[/font][/backcolor][/color][color=#0770]exit(0);
[/font][/backcolor][/color][color=#00bb]?>

<?php

class Cron extends CI_Controller {

public function __construct() {
parent::__construct();
$this->load->model('m_monitor', 'model');
}

public function test() {
// 从model中取出的数据
// $data = $this->model->getweb();
$data = array (
0 => array (
'm_name' => 'baidu',
'mid' => '7',
'm_url' => 'http://www.baidu.com'
),
1 => array (
'm_name' => 'sina',
'mid' => '8',
'm_url' => 'http://www.sina.com.cn'
)
);
foreach ($data as $key => $value) {
$pid = pcntl_fork();
if ($pid == -1) {
exit ('子进程创建失败');
} elseif ($pid) {
//pcntl_wait($status);

// model中的db方法返回的是$this->db;
// $this->model->db()->reconnect(true);
$dsn = 'dbdriver://rootlocalhost/webmon';
$this->load->database($dsn);
} else {
//当前时间戳
$time = time ();
// //上次任务运行结束时间[从model中查询数据库得到的数据]
$lasttime = $this->model->getruntime(1, $value['mid']);
// //用户设置的任务运行周期[从model中查询数据库得到的数据]
$cron_time = $this->model->gettasktime(1, $value['m_url']);
}
}
}

}

?>

<?php
// Create the MySQL connection
$db = mysql_connect ($server, $username, $password);

$pid = pcntl_fork();

if ( $pid == -1 ) {
// Fork failed
exit (1);
} else if ( $pid ) {
// We are the parent
// Can no longer use $db because it will be closed by the child
// Instead, make a new MySQL connection for ourselves to work with
$db = mysql_connect ($server, $username, $password, true);
} else {
// We are the child
// Do something with the inherited connection here
// It will get closed upon exit
exit (0);
?>