latest threads by specific user with external.php

Maybe you could try:

<script type="text/javascript" src="external.php?type=js"></script> 
<script language="" type="text/javascript">
<!--
for (var x in threads)
{
    if(threads[x].userid == xx)
   {
       document.writeln("<img class=\"inlineimg\"      src=\"$stylevar[imgdir_button]/lastpost.gif\" alt=\"\" border=\"0\" /> <a href=\"showthread.php?t="+threads[x].threadid+"\">"+threads[x].title+"</a> <span class=\"time\">($vbphrase[posted_by]: "+threads[x].poster+")</span><br  />");
   }
}

//-->
</script></div>

Though I am not sure why you would want to do this? Also, I don't have the code available to me, so please test the code, as I am not sure threads[x].userid is a valid variable off the top of my head.