The following code checks an IP address (defaults to the visitors IP address) with 81 RBL servers worldwide!

<?php
$rbl = array(
'asiaspam.spamblocked.com', 'bl.deadbeef.com',
'bl.emailbasura.org', 'bl.spamcop.net',
'blackholes.five-ten-sg.com', 'blacklist.woody.ch',
'bogons.cymru.com', 'cbl.abuseat.org',
'cdl.anti-spam.org.cn', 'combined.abuse.ch',
'combined.rbl.msrbl.net', 'db.wpbl.info',
'dnsbl-1.uceprotect.net', 'dnsbl-2.uceprotect.net',
'dnsbl-3.uceprotect.net', 'dnsbl.abuse.ch',
'dnsbl.ahbl.org', 'dnsbl.cyberlogic.net',
'dnsbl.inps.de', 'dnsbl.njabl.org',
'dnsbl.sorbs.net', 'drone.abuse.ch',
'duinv.aupads.org', 'dul.dnsbl.sorbs.net',
'dul.ru', 'dyna.spamrats.com',
'dynip.rothen.com', 'eurospam.spamblocked.com',
'fl.chickenboner.biz', 'http.dnsbl.sorbs.net',
'images.rbl.msrbl.net', 'ips.backscatterer.org',
'isps.spamblocked.com', 'ix.dnsbl.manitu.net',
'korea.services.net', 'lacnic.spamblocked.com',
'misc.dnsbl.sorbs.net', 'noptr.spamrats.com',
'ohps.dnsbl.net.au', 'omrs.dnsbl.net.au',
'orvedb.aupads.org', 'osps.dnsbl.net.au',
'osrs.dnsbl.net.au', 'owfs.dnsbl.net.au',
'owps.dnsbl.net.au', 'pbl.spamhaus.org',
'phishing.rbl.msrbl.net', 'probes.dnsbl.net.au',
'proxy.bl.gweep.ca', 'proxy.block.transip.nl',
'psbl.surriel.com', 'rbl.interserver.net',
'rdts.dnsbl.net.au', 'relays.bl.gweep.ca',
'relays.bl.kundenserver.de', 'relays.nether.net',
'residential.block.transip.nl', 'ricn.dnsbl.net.au',
'rmst.dnsbl.net.au', 'sbl.spamhaus.org',
'short.rbl.jp', 'smtp.dnsbl.sorbs.net',
'socks.dnsbl.sorbs.net', 'spam.dnsbl.sorbs.net',
'spam.rbl.msrbl.net', 'spam.spamrats.com',
'spamlist.or.kr', 'spamrbl.imp.ch',
't3direct.dnsbl.net.au', 'tor.ahbl.org',
'tor.dnsbl.sectoor.de', 'torserver.tor.dnsbl.sectoor.de',
'ubl.lashback.com', 'ubl.unsubscore.com',
'virbl.bit.nl', 'virus.rbl.jp',
'virus.rbl.msrbl.net', 'web.dnsbl.sorbs.net',
'wormrbl.imp.ch', 'xbl.spamhaus.org',
'zen.spamhaus.org'
);

$ip = $_REQUEST['ip'];

if (!filter_var($ip, FILTER_VALIDATE_IP)) {
    if (isset($_SERVER['HTTP_CLIENT_IP'])) {
        $ip = $_SERVER['HTTP_CLIENT_IP'];
    } else if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
        $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
    } else if (isset($_SERVER['HTTP_X_FORWARDED'])) {
        $ip = $_SERVER['HTTP_X_FORWARDED'];
    } else if (isset($_SERVER['HTTP_FORWARDED_FOR'])) {
        $ip = $_SERVER['HTTP_FORWARDED_FOR'];
    } else if (isset($_SERVER['HTTP_FORWARDED'])) {
        $ip = $_SERVER['HTTP_FORWARDED'];
    } else if (isset($_SERVER['REMOTE_ADDR'])) {
        $ip = $_SERVER['REMOTE_ADDR'];
    } else {
        die('Unknown IP Address');
    }
}
echo '<h1>Checking '.$ip.'</h1>';
foreach ($dnsbl_lookup as $host) {
    if (checkdnsrr($ip . "." . $host . ".", "A")) {
        echo "Listed in " . $host . "<br />";
    } else {
        echo "Not Listed in " . $host . "<br />";
    }
}