Not since PHP8 it's not [0], it's a real part of the language that you can read and view using reflection
[0] https://www.php.net/manual/en/language.attributes.overview.p...
[0] https://www.php.net/manual/en/language.attributes.overview.p...
Unknown attributes are also ignored silently, which isn't really a good sign.
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).
> `#[foo` used to be legal PHP 7, but is illegal in PHP 8).
So it is not a comment in PHP8 then.