Thanks for all the tips. For posterity, this is what I did to get it to work:
.htaccess:
>RewriteEngine On
RewriteBase /
RewriteRule ^redirecturl1\.aspx$ redirect_script.php [QSA]
RewriteRule ^redirecturl2\.aspx$ redirect_script.php [QSA]
RewriteRule ^redirecturl3\.aspx$ redirect_script.php [QSA]
redirect_script.php:
><?php
$id= ( isset( $_GET['ID'] ) ) ? $_GET['ID']: '';
$site_url = 'http://www.xxxxxxxxx.com/';
$url_redirects = array(
'1' => 'archives/URL1',
'2' => 'archives/URL2',
'6' => 'archives/URL3',
'7' => 'archives/URL4',
);
if ( $id== '' )
{
Header ("HTTP/1.1 301 Moved Permanently");
Header('Location: '.$site_url.'index.php');
}
else
{
$url = $site_url.$url_redirects[$id];
Header ("HTTP/1.1 301 Moved Permanently");
Header('Location: '.$url);
}
?>