This is one of the reasons I like PHP, in newer PHP versions it is an attribute but in older versions it is just a comment. Clever and backwards compatible.
It isn't backwards compatible (`#[foo` used to be legal PHP 7, but is illegal in PHP 8).
It may technically be forwards compatible syntactically (`#[foo]` will, as you say, just be ignored by PHP 7), but that's an anti-feature (assuming that the attribute isn't a no-op, it'll presumably break something else).
It may technically be forwards compatible syntactically (`#[foo]` will, as you say, just be ignored by PHP 7), but that's an anti-feature (assuming that the attribute isn't a no-op, it'll presumably break something else).