Я думаю, что вам не хватает -y
в команде
# yum list installed | grep php-xml
# yum info php-xml
Available Packages
Name : php-xml
Arch : x86_64
Version : 5.3.3
Release : 40.el6_6
Size : 106 k
Repo : updates
Summary : A module for PHP applications which use XML
URL : http://www.php.net/
License : PHP
Description : The php-xml package contains dynamic shared objects which add support
: to PHP for manipulating XML documents using the DOM tree,
: and performing XSL transformations on XML documents.
# yum install -y php-xml