Alternative for storing id in hidden input
I would question the premise.


  But with this, just a change to the id may affect other pending comment, or those that are already approved.


I don't see a problem here.

Given the admin is allowed to approve any comment, and nobody else is authorized to access this page completely, I don't see how it's even a problem.

To put it bluntly, nobody in the world is using such a clumsy approach as it's either useless or has a completely different solution (to ensure that a user cannot access other users' records there should be an ACL system in the database, not some HTML/Session ratatouille).