this is a good question i had.. i was able to find this: 

<?php
if (isset($_GET['width']) AND isset($_GET['height'])) {
  // output the geometry variables
  echo "Screen width is: ". $_GET['width'] ."<br />n";
  echo "Screen height is: ". $_GET['height'] ."<br />n";
} else {
  // pass the geometry variables
  // (preserve the original query string
  //   -- post variables will need to handled differently)

  echo "<script language='javascript'>n";
  echo "  location.href="${_SERVER['SCRIPT_NAME']}?${_SERVER['QUERY_STRING']}"
            . "&width=" + screen.width + "&height=" + screen.height;n";
  echo "</script>n";
  exit();
}
?>