Fix an undefined class error running gen_stub in php8

Fix an undefined class error running gen_stub in php8

For whatever reason, php 8 would not have loaded the subsequent classes when
running `php build/gen_stub.php path/to/filename.php`.
I assume it didn't load the classes immediately because there's a possibility
the code before it would throw.
(Probably because __toString was added recently and prevents early binding)

Also, fix a typo

Closes GH-5369