Fastest way to compare directory state, or hashing for fun and profit

You can be notified of filesystem modifications using the inotify extension.

It can be installed with pecl:

pecl install inotify

Or manually (download, phpize && ./configure && make && make install as usual).

This is a raw binding over the linux inotify syscalls, and is probably the fastest solution on linux.

See this example of a simple tail implementation: http://svn.php.net/viewvc/pecl/inotify/trunk/tail.php?revision=262896&view=markup

If you want a higher level library, or suppot for non-linux systems, take a look at Lurker.

It works on any system, and can use inotity when it's available.

See the example from the README:

$watcher = new ResourceWatcher;$watcher->track('an arbitrary id', '/path/to/views');$watcher->addListener('an arbitrary id', function (FilesystemEvent $event) {    echo $event->getResource() . 'was' . $event->getTypeString();});$watcher->start();

Instead of going by file contents, you can use the same technique with filename and timestamps:

find . -name '.svn' -prune -o -type f -printf '%m%c%p' | md5sum

This is much faster than reading and hashing the contents of each file.

Insteading of actively searching for changes, why not getting notified when something changes. Have a look at PHP's FAM - File Alteration Monitor API

FAM monitors files and directories, notifying interested applications of changes. More information about FAM is available at » http://oss.sgi.com/projects/fam/. A PHP script may specify a list of files for FAM to monitor using the functions provided by this extension. The FAM process is started when the first connection from any application to it is opened. It exits after all connections to it have been closed.

CAVEAT: requires an additional daemon on the machine and the PECL extension is unmaintained.