It is security realted. http://en.php.net/manual/en/features.safe-mode.php We recommend to keep open_basedir and safe_mode disabled since this may effects some eZ Publish functionality (e.g caching).
i.e a colon-separated list of allowed paths, and symlink the binaries you want to allow ezpublish to access in /usr/local/ezpublish/bin (or whichever path you want).