Instalando os repositórios:
rpm -Uvh http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-4.noarch.rpm
rpm -Uvh http://dl.iuscommunity.org/pub/ius/stable/Redhat/5/i386/ius-release-1.0-6.ius.el5.noarch.rpm
Caso exista alguma versão de PHP instalada execute:
yum remove php php53 php-* php53-*
Instalando o PHP 5.3:
yum install php53u-pear php53u php53u-cli php53u-common php53u-devel php53u-gd php53u-mbstring php53u-mcrypt php53u-mysql php53u-pdo php53u-soap php53u-xml php53u-xmlrpc php53u-bcmath php53u-pecl-apc php53u-pecl-memcache php53u-snmp
