javascript value to php

Question

Because I want use javascript value (lat and lon) sent to php, but I don't know why php always is null? Please assist me fix the issue

function getElem(id) {
  return typeof id === 'string' ? document.getElementById(id) : id;
}

function show_it(lat, lon) {


  var str = '您当前的位置，纬度：' + lat + '，经度：' + lon;
  getElem('geo_loc').innerHTML = str;

}

if (navigator.geolocation) {
  navigator.geolocation.getCurrentPosition(function(position) {
    show_it(position.coords.latitude, position.coords.longitude);
  }, function(err) {
    getElem('geo_loc').innerHTML = err.code + "\n" + err.message;
  });
} else {
  getElem('geo_loc').innerHTML = "您当前使用的浏览器不支持Geolocation服务";
}

PHP:

<?php 
   $dummy = "<script>document.write(ja)</script>";
   echo $dummy; 
 ?>