Enable zip on PHP 5.1.6 without recompile-Collection of common programming errors

That your user check their software, if is custom made, that use the zip_* functions instead the ZipArchive class, if not, that try to search a version that support PHP 5.1.x.

Upgrade your php in your CentOS server to 5.2, you can use IUS Community repo http://iuscommunity.org/, remi repo http://blog.famillecollet.com/pages/Config-en CentAltRU, http://centos.alt.ru for this (and test that your other php sofware dont get affected)