Fix Error: Fatal error: Call to undefined function dom_import_simplexml() in /var/www/vhosts/www.yoursite.com/updates/concrete188.8.131.52/concrete/core/controllers/single_pages/upgrade.php on line 182
While trying to upgrade concrete5 5.2.1 to 184.108.40.206
You are probably missing a phpXML library.
On Scientific Linux I fixed this by running the following commands.
1. sudo yum install php-xml php-xmlrpc -y
2. service httpd restart
Navigate back to your "upgrade" page.
The upgrade should complete successfully.