PHP, symlinks, and the realpath cache
I've had issues in the past where modifying symlinks seems to break my web
apps: I change the symlink target, but PHP still loads the old target for
seconds or minutes. Yesterday I realized this was happening because of the PHP
realpath cache. This cache can be emptied using
clearstatcache(), but the cache is maintained per-thread,
so it's not a silver bullet. Only the active thread will have its cache
cleared, where you may have dozens of threads in total (e.g. instances of
php-fpm, or httpd children in prefork mode).
So, from now on I'll just consider symlinks harmful, I suppose.