My PHP error logs have been filling up with an ominous error message:
PHP Warning: main(): php_network_getaddresses: getaddrinfo failed: Temporary failure in name resolution (is your IPV6 configuration correct? If this error happens all the time, try reconfiguring PHP using --disable-ipv6 option to configure)
I don't know about the low-level cause for the problem, but I've figured out why it's happening so often on my server. I had the bad habit of using URLs in include statements for local pages, like this:
This was causing some pages to require three or more extra domain name lookups, just to pull files from my own server! I'm switching these statements to local file references:
Server performance has improved noticeably, though I still have no idea what any of this has to do with IPV6.