It turned out that php-posix was not installed. In order to be lazy, yum was used to install components directly.
System: centos 6.2 php version: PHP 5.1.6
1. Install php-posix
#yum -y install php-process
2. Verify that
#php -m|grep posix
posix is installed
, you can see that php already supports posix, restart apache and it will be ok.