read javascript cookie with PHP on the same page

I have a page, return.php. It has a line of HTML as follows:

<td><a href="#" dataP="<?= $partNumber[$x] ?>" onclick="popup(this)"><?= substr($avgCost[0],0,6) ?></a></td>

onclick, this calls the following popup function:

function popup(windowname) {  
  document.cookie = 'popupPartNumber='+windowname.getAttribute("dataP")+'; expires=Wed, 1 Jan 2070 13:47:11 UTC; path=/'
  var x = document.cookie;
  console.log(x);    
  windowname = 'popUpDiv';
  blanket_size(windowname);
    window_pos(windowname);
    toggle('blanket');
    toggle(windowname);

}

on this same page, this loads:

<div id="blanket" style="display:none;"></div>
  <div id="popUpDiv" style="display:none;"">
  <a href="#"" onclick="popup(this)"><img src="/img/close.png"></a>
<?php
  $popupPartNumber  = $_COOKIE['popupPartNumber'];
  echo "<pre>";
  echo htmlentities($_COOKIE['popupPartNumber'], 3, 'UTF-8');
  //echo "cookie is: " . $_COOKIE['popupPartNumber'];
  echo "</pre>";
?>

</div>

When I start this out and I click on the HTML link, I get a console.log output that says:

PHPSESSID=cvbt1gvisul27nccenaeum1hh0; popupPartNumber=123456789

but my php echo output says: null

Why is the cookie that I'm setting correctly in javascript not available to php?