PHP Zip Radius Help

Hey all I got a problem. I am trying to figure out this Zip radius script that I have. Right now you can enter in a zip code and it will find all other zipcodes in a radius of 'X' miles and order them by distance. I am trying to figure out a way to have another query statement that checks to see if any of the ZIPcodes found exist in a table called users in a field called user_zip and if so it spits out those entries instead. Here is the code I have been using for the search. Any help with this is very much appreciated. Thanks

<?php
$db_Hostname = "localhost";
$db_Database = "database";
$db_UserName = "user";
$db_Password = "pass";
$set["db_table"] = "zip";

if(!trim($_POST[zipcode])) $mess.="- zip code is required<br>";
elseif(!is_numeric($_POST[zipcode])) $mess.="- zip code must be a number<br>";
if(!trim($_POST[distance])) $mess.="- radius distance is required<br>";
elseif(!is_numeric($_POST[distance])) $mess.="- radius distance must be a number<br>";

mysql_connect($db_Hostname, $db_UserName, $db_Password) || die("Can't Connect to Database: ".mysql_error());
mysql_select_db($db_Database);

if(trim($mess)) {
echo "<p>$mess</p>";
} else {
$z=RadiusSearch($_POST[zipcode],$_POST[distance],$_POST[unit]);
$tot = count($z);
$output = $z[$i]["zip"];
if ($_POST[unit]== 0) { $_POST[unit] = "miles"; } else { $_POST[unit] = "KM"; }
$array.="<table width='620' border='0' cellspacing='1' cellpadding='4' bgcolor='#CCCCCC' align='center'>";
if(is_array($z) && $tot) {
$array.="</tr>";
for ($i=1;$i<count($z);$i++) {
$array.="<tr bgColor='#FFFFFF'>
<td>".$z[$i]["zip"]."</td>
</tr>";
}
}
echo $array."</table>";
}

function RadiusSearch($zipcode,$distance,$unit) // zip code, distance (KM), units(0 for miles, 1 for KM)
{
global $set;
// This query statement finds the zip code that was entered
$q=mysql_query("SELECT zip,Latitude,Longitude,sinlat,coslat,lonrad FROM $set[db_table] WHERE zip=$zipcode") or die(mysql_error());
if(mysql_num_rows($q)) {
$r=mysql_fetch_array($q);

// This if statement checks to see if the unit is in miles or KM
if ($unit==0) {
//$cosd is equal to the appx. earth circumfrance times acos(sinlat x sinlat of the user zipcode + coslat x coslat of the user zipcode x the cos oflonrad - the lonrad of the user zipcode
$cosd = "3963.1676*acos(sinlat*$r[sinlat]+coslat*$r[coslat]*cos(lonrad-$r[lonrad]))";
} else {
$cosd = "6378.1*acos(sinlat*$r[sinlat]+coslat*$r[coslat]*cos(lonrad-$r[lonrad]))";
}

// This query statement finds all other zip codes within the given radius
$q=mysql_query("SELECT zip, city, county, state, latitude, longitude, ROUND($cosd, 1) AS dist FROM $set[db_table] WHERE $cosd <= $distance and zip<>$zipcode ORDER BY dist") or die(mysql_error());
if(mysql_num_rows($q)) {
// Looping array spits out all the zip codes with in the radius and orders them by distance
while($r=mysql_fetch_row($q)) {
$array[$i]["zip"] = $r[0];
$array[$i]["distance"] = $r[6];
$i++;
}
return $array;
} else { echo "Sorry! No result found!"; exit; }
} else { echo "Invalid zip code!"; exit; }
}
?>