Proposal for serious BC compatibility aka language versioning

Hi, So, if we are talking about PHP 6, then the opening <?php tag should be accompanied by the version it was written for, it was tested with and then the engine could switch to a compatibility mode for that version. Seems to me that this would solve the problem where a host can't upgrade to a more modern version for fear of breaking old code. Consequently, open source packages -- like Drupal I am deeply involved with -- can't use more modern PHP versions decreasing the "push" for hosts to upgrade. This is a devilish circle and it'd be great to break it. Regards, Karoly Negyesi