PHP: Call to undefined function: simplexml_load_string()

For PHP 7 and Ubuntu 14.04 the procedure is follows. Since PHP 7 is not in the official Ubuntu PPAs you likely installed it through Ondřej Surý's PPA (sudo add-apt-repository ppa:ondrej/php). Go to /etc/php/7.0/fpm and edit php.ini, uncomment to following line:

extension=php_xmlrpc.dll

Then simply install php7.0-xml:

sudo apt-get install php7.0-xml

And restart PHP:

sudo service php7.0-fpm restart

And restart Apache:

sudo service apache2 restart

If you are on a later Ubuntu version where PHP 7 is included, the procedure is most likely the same as well (except adding any 3rd party repository).

If the XML module is not installed, install it.

Current version 5.6 on ubuntu 14.04:

sudo apt-get install php5.6-xml

And don't forget to run sudo service apache2 restart command after it

Zulhilmi Zainudi