The installation requirements note that "PHP version 5.3.3 or later is required". Does that include PHP 7?
Topic on Project:Support desk
While I'm not a MediaWiki dev, common sense would tell me that it would include PHP 7.
It should mostly be working, with less problems in newer releases (1.26 over 1.23). In case you find a bug, you can open a bugreport, but I think it should basically be working.