1、小程序获取用户信息，直接调用app的getUserInfo方法。
2、获取用户OpenId。
小程序里面通过login得到一个code：

wx.login({
      success: function (res) {
        if(res.code) {
          wx.request({
            url: 'https://xxx/testApp.php',
            data: {
              op: 'getId',
              code: res.code
            },
            success: function(res) {
              console.log(res);
              var data = JSON.parse(res.data);
              Qx.dialog('OpenId', data.openid);
              that.setData({ openId: data.openid });
            }
          })
        }
      }
    })

服务端需要通过code去微信服务器换取openId(testApp.php)：

<?php
function post($url, $data){
    $opts = array('http' =>
        array(
            'method'  => 'POST',
            'header'  => array("User-Agent: liny", "Content-type: application/json; charset=utf-8") ,
            'content' => $data
        )
    );
    $context = stream_context_create($opts);
    $result = file_get_contents($url, false, $context);
    return $result;
}

$param = $_GET;
$op = $param['op'];
if($op = 'getId') {
    $code = $param['code'];
    $url = "https://api.weixin.qq.com/sns/jscode2session?appid=&secret=&js_code=".$code."&grant_type=authorization_code";
    $res = post($url, "");
    echo json_encode($res);
}