Description:
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First I looked here php.net/manual/en/language.oop5.typehinting.php which links to here php.net/manual/en/functions.arguments.php#functions.arguments.type-declaration

The only mention of nullable is using the old syntax: php.net/manual/en/functions.arguments.php#example-140

There's nothing on the return types page either: php.net/manual/en/functions.returning-values.php

Finally, I looked at the Types page, but nothing there either php.net/manual/en/language.types.php

I eventually found what I was looking for in the new features part of the migration guide, but feel it should be in the main docs too: php.net/manual/en/migration71.new-features.php