In case that you are running PHP on CentOS or other RHEL clone distro which the POSIX support is disabled by default, you will need the php-process package to enable it. Simply install it via the yum:
$> yum install php-process
and then restart the web server.
This module contains an interface to those functions defined in the IEEE 1003.1 (POSIX.1) standards document which are not accessible through other means.
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.
Note: This extension is not available on Windows platforms.