Noticed on wikimedia/xmp-reader, as it still supports PHP >= 5.6.99, testing on 7.0, 7.1 and hhvm. mediawiki/mediawiki-codesniffer >= 28 needs PHP 7.2
https://travis-ci.org/wikimedia/xmp-reader/builds/601743715
Problem 1 - Installation request for mediawiki/mediawiki-codesniffer 28.0.0 -> satisfiable by mediawiki/mediawiki-codesniffer[v28.0.0]. - v28.0.0 requires php >= 7.2.0 -> your PHP version (7.0.33) does not satisfy that requirement.