I\'m testing PHP7, and have a weird issue after a recent update. SimpleXML should be enabled by default, and my phpinfo page shows that it is available:
phpinfo
For Alpine (in docker), you can use apk add php7-simplexml.
apk add php7-simplexml
If that doesn't work for you, you can run apk add --no-cache php7-simplexml. This is in case you aren't updating the package index first.
apk add --no-cache php7-simplexml