The correct macro to test is PHP_MAJOR_VERSION so these two PHP 8 cases
weren't ever used, which hid that the PHP8 version of the code was
broken in one of them.
Highlighted in #2113.
Most pre-defined interfaces are accessible via zend_class_entry*
variables declared in the PHP C API - we can use these to add
an interface at MINIT time (rather than having to wait until RINIT to
look up by name) by having a mapping from PHP interface name to them.
This will also be a little faster than looking up by name.
Closes#2013