Then your understanding of the release numbering system is flawed. Going from 5.3.x to 5.4.x is a major release. Going from 5.3.x to 5.3.y is a minor release.
If you look at http://php.net/manual/en/migration54.incompatible.php you will see a whole page full of backward incompatible changes for the move from 5.3.x to 5.4.x. There are similar lists for all the other major releases.