I was playing today with some file uploading scripts and noticed that they require one of the two in order to be able to display a progress bar.
Those are the steps required to enabled both of them on centos:
yum install php-pear php-devel httpd-devel pcre-devel pecl install apc pecl install uploadprogress
Than I created in /etc/php.d/ the following files:
; provides upload progress, installed using pecl extension=apc.so apc.rfc1867=1
; provides upload progress, installed using pecl extension=uploadprogress.so
All you need to do now is restart apache!
Please note that I was using PHP 5.2 from testing at the time