具体方法
1、配置网页授权回调域名，如 www.111com.net
2、模拟公众号的第三方网页，http://www.111com.net/getcodeurl.php
if(isset($_SESSION['user'])){
print_r($_SESSION['user']);
exit;
}
$APPID='公众号在微信的appid';
$REDIRECT_URI='http://www.111com.net/callback.php';
$scope='snsapi_base';
//$scope='snsapi_userinfo';//需要授权
$url='https://open.weixin.qq.com/connect/oauth2/authorize?appid='.$APPID.'&redirect_uri='.urlencode($REDIRECT_URI).'&response_type=code&scope='.$scope.'&state='.$state.'#wechat_redirect';
header("Location:".$url);
?>
3、第三方网页的回跳url中，首先从请求中取得code，然后根据code进一步换取openid和access_token，然后就可以根据openid和access_token调用微信的相关接口查询用户信息了。
//http://www.111com.net/callback.php
$appid = "公众号在微信的appid";
$secret = "公众号在微信的app secret";
$code = $_GET["code"];
$get_token_url = 'https://api.weixin.qq.com/sns/oauth2/access_token?appid='.$appid.'&secret='.$secret.'&code='.$code.'&grant_type=authorization_code';
$ch = curl_init();
curl_setopt($ch,CURLOPT_URL,$get_token_url);
curl_setopt($ch,CURLOPT_HEADER,0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1 );
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 10);
$res = curl_exec($ch);
curl_close($ch);
$json_obj = json_decode($res,true);
//根据openid和access_token查询用户信息
$access_token = $json_obj['access_token'];
$openid = $json_obj['openid'];
$get_user_info_url = 'https://api.weixin.qq.com/sns/userinfo?access_token='.$access_token.'&openid='.$openid.'&lang=zh_CN';
$ch = curl_init();
curl_setopt($ch,CURLOPT_URL,$get_user_info_url);
curl_setopt($ch,CURLOPT_HEADER,0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1 );
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 10);
$res = curl_exec($ch);
curl_close($ch);
//解析json
$user_obj = json_decode($res,true);
$_SESSION['user'] = $user_obj;
print_r($user_obj);
?>
参考资料
网页授权获取用户基本信息
http://mp.weixin.qq.com/wiki/17/c0f37d5704f0b64713d5d2c37b468d75.html