<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>PHP+Ajax实时自动检测是否联网</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script type="text/javascript">
<!--
var xmlHttp;
function createXMLHttpRequest(){
  if(window.ActiveXObject){
    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
  }
  else if(window.XMLHttpRequest){
    xmlHttp = new XMLHttpRequest();
  }
}
function start(){
  createXMLHttpRequest();
  var url="getNetLink";
  xmlHttp.open("GET",url,true);
  xmlHttp.onreadystatechange = callback;
  xmlHttp.send(null);
}
function callback(){
  if(xmlHttp.readyState == 4){
    if(xmlHttp.status == 200){
      document.getElementById("shownetlink").innerHTML = xmlHttp.responseText;
      setTimeout("start()",8000);
    }
  }
}
// -->
</script>
</head>
<body onload="start();">
<h1>PHP+Ajax实时自动检测是否联网</h1>
<p>当前网络状态：<span id="shownetlink"></span></p>
</body>
</html>

public function getNetLink(){ 
  header("cache-control:no-cache,must-revalidate"); 
  header("Content-Type:text/html;charset=utf-8"); 
  $file=fopen("http://www.baidu.com/", "r"); 
  if (!$file){ 
   $shownetlink = "<font color=\"red\">网络连接失败</font>"; 
  }else{ 
   $shownetlink = "<font color=\"#06C\">网络连接正常</font>"; 
  } 
  echo $shownetlink; 
}