How to force wx.request request results not to use cache?

<code> wx.request({
  url: 'wxapp.php',
  data: {
     x: '12' ,
     y: '34'
  },
  header: {
      'Content-Type': 'application/json'
  },
  success: function(res) {
    console.log(res.data)
  }
})
</code>

php:

<code><?php
$x=$_GET['x'];
$y=$_GET['y'];
$z=['x'=>$x,'y'=>$y];
echo json_encode($z);</code>

Dang

<code>      data: {
         x: '12' ,
         y: '34'
      }</code>

Return

<code>1234</code>

Dang

<code>      data: {
         x: '123' ,
         y: '343'
      }</code>

Return

<code>Object {x: "123", y: "343"}</code>

Modified again

<code>      data: {
         x: '12' ,
         y: '34'
      }</code>

Compile, clear cache, restart
Return

<code>1234</code>

What is this situation?

Reply content:

<code> wx.request({
  url: 'wxapp.php',
  data: {
     x: '12' ,
     y: '34'
  },
  header: {
      'Content-Type': 'application/json'
  },
  success: function(res) {
    console.log(res.data)
  }
})
</code>

php:

<code><?php
$x=$_GET['x'];
$y=$_GET['y'];
$z=['x'=>$x,'y'=>$y];
echo json_encode($z);</code>

Dang

<code>      data: {
         x: '12' ,
         y: '34'
      }</code>

Return

<code>1234</code>

Dang

<code>      data: {
         x: '123' ,
         y: '343'
      }</code>

Return

<code>Object {x: "123", y: "343"}</code>

Modified again

<code>      data: {
         x: '12' ,
         y: '34'
      }</code>

Compile, clear cache, restart
Return

<code>1234</code>

What is this situation?

<code>wx.request({
  url: 'wxapp.php',
  data: {
     x: '12',
     y: '34',
     _: (new Date()).getTime() 　//加个随机参数
  },
  header: {
      'Content-Type': 'application/json'
  },
  success: function(res) {
    console.log(res.data)
  }
});</code>