Due to Davey Shafik's prodding, I took a look at PHP_Archive again and found a few subtle bugs which are fixed in CVS. More on this later, the bug was caused by an interesting problem in the way fread() works inside a userspace stream handler.
But for now, I'm excited to have a unique demonstration of PHP_Archive's power. The file http://pear.chiaraquartet.net/phpmyadmin.phar (Note: right-click and save) is a single-file bundling of the popular phpMyAdmin application. The only thing it needs to function properly is a config.inc.php file. A template file is auto-generated on the first time it is run, so running:
$ php phpmyadmin.phar
will spit out the config.inc.php needed. All you need to do is modify the user/password and the kind of extension (mysql or mysqli) and it can be used. To use it from a website, either add .phar to the extensions parsed by PHP, or simply rename it to phpmyadmin.php. As always, PHP 5.1 or newer is required in order to run .phar files.