<?php

function load_js($localPath) {
  if (file_exists($localPath)) {
    echo '<'.'script language="JavaScript" src="'.htmlspecialchars($localPath.'?'.filemtime($localPath)).'"><'.'/script>';
  } else {
    user_error("Unknown file:'$localPath'");
    exit;
  }
}

load_js('test.js');
load_js('there-is-no-such-file.js');

?>

...