All true, but practically speaking I don't know how complete the spec needs to be. Looking at the HHVM project, they fail a lot of the basic PHP unit tests, yet

All true, but practically speaking I don't know how complete the spec needs to be. Looking at the HHVM project, they fail a lot of the basic PHP unit tests, yet also have 100% parity with nearly every major CMS/framework there is. PHP is just old, and as gone without a spec so long that there's a whole pile of code that nobody ever touches anymore. Simply implementing a subset of the spec, like HHVM has, is really all I need.