In Ubuntu, libraries are actually included in either the main binary package or a lib* variant. The -dev package is usually just a pile of .h include files (text files), that aren't linked against anything. When the package maintainer builds the mysql package, it actually builds all the different sub-packages (mysql-client, mysql-server libmysqlclient, libmysqlclient-dev) at the same time and as such would have all of the updates pushed out simultaneously.

What you are suggesting would be near impossible to have happen without the user manually downloading deb packages, force installing using dpkg, and even then apt would shit itself over conflicting packages and broken dependencies. If someone manages to actually do that... well, they're going to have bigger problems than just "PHP isn't working right."

Rarely do packages need a particular kernel version, just ask anyone using xen or openvz where the kernel is often quite old (2.6.18) and not quite as easily upgradeable. The only time I've known of conflicts arising are when you need to use 3rd party binary drivers, or when an application is using bleeding edge kernel calls which is usually a bug upstream.

You may have a small point with glibc, but I imagine they'll just pin it to a major update every 3 to 6 months and give themselves time to test the heck out of it.