Users of the PHP_Archive PEAR package should check out Greg Beaver's latest post for some information about the latest version of the package:

I just pushed out a release of PHP_Archive 0.9.1. This release fixes a bug that affects 64-bit users of PHP 5.2 and newer. Basically, the unpack() function does not behave in a predictable way when unpacking large integers. This is a known bug, and is documented at the manual page, although I was unaware of the issue until today.

He gives the example code that would cause the problem and a more correct version that would allow the package to work with all versions of PHP (previously, it would fail due to a bad return from pack). The latest version of the package can be grabbed from it PEAR page or via the "pear" command to grab it automatically.