wget http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-3.noarch.rpm
    wget http://rpms.famillecollet.com/enterprise/remi-release-5.rpm
    rpm -Uvh remi-release-5*.rpm epel-release-5*.rpm
    yum --enablerepo=remi update php

php -v
 
PHP 5.3.0 (cli) (built: Jul 19 2009 17:55:08)
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2009 Zend Technologies

if you got dependency issue, you should also update mysql as same time use this command

    yum --enablerepo=remi update php mysql