PHP manual for POSIX reads:
Warning - Sensitive data can be retrieved with the POSIX functions, e.g. posix_getpwnam() and friends. None of the POSIX function perform any kind of access checking when safe mode is enabled. It's therefore strongly advised to disable the POSIX extension at all (use --disable-posix in your configure line) if you're operating in such an environment.
You need POSIX for installing Silverstripe as I understand it... So, this maybe something to check as a requirement for your webhost ( I found that www.godaddy.com did not offer this).
Can anyone confirm this theory?