PHP:: BlackList Lookup

For doing basic RBL (Real Time Blacklist) lookups with this function do:

<?
$host = '64.53.200.156';
$rbl  = 'sbl-xbl.spamhaus.org';

// valid query format is: 156.200.53.64.sbl-xbl.spamhaus.org
$rev = array_reverse(explode('.', $host));
$lookup = implode('.', $rev) . '.' . $rbl;

if ($lookup != gethostbyname($lookup)) {
    echo "ip: $host is listed in $rbl\n";
} else {
    echo "ip: $host NOT listed in $rbl\n";
}
?>