We should query and finalise what version of PHP to support... Currently we have 7.0.0 in composer.json, that's going to be changed...
https://github.com/wikimedia/mediawiki/blob/56e2567/composer.json#L35
https://github.com/wikimedia/mediawiki/blob/ed8e939/includes/PHPVersionCheck.php#L98
Should we consider to use/set 7.2.9 (rather than 7.2.0) to match and not conflict with symfony's requirements?
Ping T216166