Assign javascript value to php variable

I'm trying to check screen width and assign it to a php variable to do some if else statements. This is what I got.

            $widthChecked = "";
        echo $widthChecked;
    ?>

This works perfectly. But when I try to echo something based on the mobileFormReady value, it doesnt echo.

This is what im trying to get to work.

$widthChecked = "";
if($widthChecked == "22"){
  echo 'this page is under 767 pixels';
}

else if($widthChecked == "55"){
  echo 'this page is OVER 767 pixels';
}
else{
  echo 'NOT WORKING YET';
}

I think its a string integer issue. But I cant seem to figure it out. Can you guys please help me? Thanks a lot.