Individual hard PHP limits for each hosting package

As a web hosting provider I would like to be able to set PHP limits (like memory_limit, upload_max_filesize etc) that:

1. can't be changed by the user with a local php.ini file, with cronjob directives or with ini_set.

2. can be implemented in a per package specific fashion (e.g. smaller packages get smaller limits and bigger packages get bigger limits).

Limiting for instance memory_limits to say 256MB would allow the vast majority of sites to work flawlessly but would prevent the usage of a hungry PS site (min 768MB memory_limit in recent versions) inside a very small hosting package.