Ajax GET and POST method

Ajax GET and POST method

I am currently using ajax GET method to pass my value into the same page.
I look at the console.log, the requestURL is able to pass the value like www.example.com/test?q=1. 
unfortunately,when it request by the server site, the get value is not assigned to the variable $_GET['q']; As a result, i couldn't manipulate the value after the request. 

Do you have any idea what've I missed ? or it is fundamentally not allow to do in the same page.

$anyvar = $_GET['q'];
  2. function showData(str)
  3. {
  4.   bool = false;
  5.   var str1 = "";
  6.   if(str == "")
  7.   {
  8.     document.getElementById("select_field").innerHTML = "";
  9.     return;
  10.   }
  11.   else
  12.   {
  13.     
  14.     if(window.XMLHttpRequest)
  15.     {
  16.       xmlhttp = new XMLHttpRequest();
  17.       console.log(xmlhttp);
  18.     }
  19.     else
  20.     {
  21.       xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
  22.       console.log(xmlhttp);
  23.     }
  24.     xmlhttp.onreadystatechange = function()
  25.     {
  26.       if(xmlhttp.readyState == 4 && xmlhttp.status == 200)
  27.       {
  28.         document.getElementById("select_field").innerHTML=document.getElementById("selection").innerHTML;
  29.       }
  30.     };
  31.     
  32.   }
  33.   xmlhttp.open("GET", "reports.php?q=" + str, true);
  34.   xmlhttp.send();
  35. }