"Phan" is a static analyser for PHP: https://github.com/etsy/phan
MediaWiki continuous integration (see https://www.mediawiki.org/wiki/Continuous_integration ) now runs Phan and highlighted a lot of issues, which have been temporarily suppressed (so they will not be shown) until those issues are fixed in the MediaWiki core code repository itself.
Fix 10-20 errors and submit your patch in Gerrit! :)
(And when all issues of a certain type are removed from MediaWiki core, they should be removed from `suppress_issue_types`
* PHP 7+
* php-ast extension (packages for Ubuntu ≥ 16.04 at http://packages.ubuntu.com/yakkety/php-ast ; packages for Fedora ≥ 25 at https://koji.fedoraproject.org/koji/packageinfo?packageID=23350 )
** For others, including Windows and building from source see https://github.com/nikic/php-ast/blob/master/README.md#installation