Is it even worth pretending to support PHP 5.3 at this point?
WP_Mock@dev-dev is required as a dev dependency - As of the latest commit to soter-core this put us at ~0.2.x which required PHP5.6+. Currently, it would pull ~0.3.x which requires PHP7+.
All that to say that it is unlikely this package will ever be properly tested below PHP5.6 going forward, and maybe not even below 7.0.
https://github.com/ssnepenthe/soter-core/blob/b63bfc54d370d1b73261acb593ec315c1634ea07/src/class-response.php#L229-L231
Is it even worth pretending to support PHP 5.3 at this point?
WP_Mock@dev-dev is required as a dev dependency - As of the latest commit to soter-core this put us at ~0.2.x which required PHP5.6+. Currently, it would pull ~0.3.x which requires PHP7+.
All that to say that it is unlikely this package will ever be properly tested below PHP5.6 going forward, and maybe not even below 7.0.
Alternatively - consider introducing a tool such as https://github.com/wimg/PHPCompatibility to avoid simple issues such as this in the future.