Buildkit Site fails to load due to PHP ugrade from 5 to 7
It's on your php.ini config file. where it it depends on your OS, in ubuntu/debian it's under /etc/php5 and you have cli and fpm, check that the cli version enables the extension simplexml

FYI, CiviCRM is not yet super tested with php7 (was classified "experimental" two months ago

https://civicrm.org/blog/totten/civicrm-4712-rc-experimental-support-for-php-7-mysql-57

for the details: https://issues.civicrm.org/jira/browse/CRM-17789

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