I am trying to connect 2 select boxes together.

What I have in mind is this: The user chooses a userid from the first select box. Once they select the userid it projects data based off only the userid.

There is a second select box that holds year values so that once a year is selected it will filter the data of that userid to only show for that specific year.

I'm not that well with Javascript so I don't know what exactly I'm trying to do in the Javascript area.
HTML Code:
<script type="text/javascript">
function saveUser(userid)
{

var userid;
showUser(userid);

}

function saveYear(year)
{
var year;
showUser(year);
}

function showUser(userid,year)
{
if (userid =="" && year =="")
  {
  document.getElementById("txtHint").innerHTML="";
  return;
  }
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","Status_23Gs.php?q="+userid+"&year="+year,true);
xmlhttp.send();
}
</script>
*******Select Box1*******
PHP Code:

<?php

$query
="SELECT uuid,auth,emp FROM auto_atp.Admin";
$result=mysql_query($query); 


$options="";
while(
$row mysql_fetch_array($result)){


    
$id=$row['uuid'];
    
$users=$row['uuid'];
    
$options.="<OPTION VALUE=\"$id\">".$users;
}
?>

<b>UUID:</b><select id="theuser" name="users" onchange="saveUser(this.value)">
<OPTION VALUE=0><b>UID</b>
<?=$options?>
</SELECT>
PHP Code:
*****Select Box 2*******
<?php
$query
="SELECT serial,year FROM auto_atp.GeoProbe GROUP BY year ORDER BY year";
$result=mysql_query($query); 


$options="";
while(
$row mysql_fetch_array($result)){

    
$RES=$row['year'];    

    
$options.="<OPTION VALUE=\"$RES\">".$RES;

}
?>

<select id="theyear" name="year" onchange="saveYear(this.value)">
<OPTION VALUE=0>

<?=$options?>
</SELECT>
****OUTPUT********
<div id="txtHint"><b></b></div>