implied assignment of license

Indeed, the PHP License's upgrade clause makes this much more sound in my opinion as well.

To your point, there is perhaps a precedence here that actually involved the FSF and a third-party license! This happened in the relicensing of Wikipedia the GFDL to CC-BY-SA. This happened in 2009, by virtue of Wikipedia being licensed at the time as GFDL 1.2 "or any later version'. The FSF graciously released a new version of GFDL (at the request of the Wikimedia Foundation), v1.3, that had a carve-out that allowed relicensing to the CC-BY-SA. So the path there was GFDL 1.2 -> GFDL 1.3 -> CC-BY-SA 3.0 -> (eventually) CC-BY-SA 4.0. Each license allowed relicensing to the next one.

In the same sense, I think the PHP Group could have released PHP License 3.02, that added a new Relicensing clause that allowed relicensing to the BSD-3-clause. It seems like they skipped this step, but I suppose that does not make any meaningful difference.

IANAL, but Pam Chestek is and an excellent one, so I'm sure whatever they're doing is legally sound :)