Currently the PHP coding standard is only enforced through a pre-commit hook and things might be missed.
This step should be enforced by the CI.
PHPCS [0] is not able to silence an existing set of issue in the codebase. As such, and to avoid too much false-positive, the sane approach [1] seems to progressively add rule to enforce to fix the corresponding issues all over the codebase.
The existing pre-commit hook will be kept as is to try to do the best it can to detect issues and the CI will enforce a subset of the ruleset.
To test the approach, a new coding standard rule is going to be added: the unused use in the PHP files will now be rejected.
[0]
https://github.com/squizlabs/PHP_CodeSniffer/
[1]
https://github.com/squizlabs/PHP_CodeSniffer/issues/2094#issuecomment-409688782