I think the OP's point was that python 3 was never intended to be backwards compatible and intentionally broke compatibility in several places. For this reason, 2.7 continued to be supported for quite some time. I don't know a lot about PHP, but I'm guessing this was not the case for their minor version updates.
PHP changes were mostly, but not completely backwards compatible. There were a few corner cases were you'd get problems, but the majority of code would run fine on both systems.
