PHP JSON data passing

Just some edits to Louys Patrice Bessette code to make this work. Notice though the request here is a POST request.

test.html

<input type="checkbox" class="pushchecks" value="JAN" >
<input type="checkbox" class="pushchecks" value="FEB" >
<input type="checkbox" class="pushchecks" value="MAR" >

<!--to readable output from the php file-->
<pre class="result"><pre>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script> 
<script>
$('body').on('click','.pushchecks',function(){           
         // An array.
         var bills=[];

  // Loop trought all .pushchecks checked.
  // Edited to be .pushchecks instead of .checks
  $(".pushchecks:checked").each(function(){

    // Add this value in array.
    bills.push( $(this).val() );
  });

  // Stingify the array. Would convert the array to json object
  bills = JSON.stringify(bills);

  // Ajax request to load process.php result in the pre tags above.
  // Use $('*') to select all elements in the DOM
  $('.result').load('process.php', { checkedbills:bills }, '.list',function(){});
});
</script>

process.php

<?php

 $billsAry  =   json_decode($_REQUEST['checkedbills']);
 print_r($billsAry);  //The response and output to the request 
?>

You aren't far...
The thing is that your loop was keeping the last checked value only.

Using an arry, you can make sure you collect all values.

$('body').on('click','.pushchecks',function(){

  // An array.
  var bills=[];

  // Loop trought all .pushchecks checked.
  $(".checks:checked").each(function(){

    // Add this value in array.
    bills.push( $(this).val() );
  });

  // Stingify the array.
  bills = JSON.stringify(bills);

  console.log(bills);    // Would print : {["JAN",MAR"]}

  // Ajax.
  $.load('process.php', { checkedbills:bills }, '.list',function(){});
});