PEAR has had PHP support for SURBLs and other blacklists since January 2005 via the Net_DNSBL package:

http://pear.php.net/manual/en/package.networking.net-dnsbl.php

but a very early version of Net_DNSBL did not properly cache DNS queries. If you are using PEAR's Net_DNSBL, please make sure you're using a recent version.

Given that PEAR is a widely used PHP component framework, it's possible Net_DNSBL queries are built into many different applications. Any recent applications should be fine, but it may be worth checking any old versions for updates or to generally update your PEAR versions.