This extension helps to write documents from XML format to Text. And this will be enabled by default. If such extension is not available, make use of below command to install and use.

  # yum install libxml2
  # service httpd restart
  # service php-fpm restart

Once its installed go to /etc/php.d/xmlwriter.ini file and remove the hashed line as follows.

; Enable xmlwriter extension module
extension=xmlwriter.so