$ips = array(
    "a" => array( 'xxx.xxx.xxx.xxx', ....),
    "b" => array( 'xxx.xxx.xxx.xxx', ....),
);

if(in_array($ips['a'], $_SERVER['REMOTE_ADDR'])) {
    header("location: website.com/a");
} else {
    header("location: website.com/b");
}
exit;