Thanks for the PR. I tried it on Symfony and I get tons of deprecations, of course.
It would be great to fix them one by one to get a better understanding of the impact this PR could have on userland.

I'm already wondering if we shouldn't discuss about both directions of the conversion separately. It looks more of a smell to accept a bool where eg a string is expected, than to accept a string where a bool it required.

This maps to structs like if ($boo) {...}. If this is accepted for any types (as it should), then the conversion rules are plain explicit and well known for the to-bool direction.

Places I found where the direction is from-bool are way less legit at first glance.

Can you try running the test suite? Here is the doc about how to: https://symfony.com/doc/current/contributing/code/tests.html

Then look at random deprecations and try to fix some to get some taste of the deprecation :)

Thanks for the PR. I tried it on Symfony and I get tons of deprecations, of course. It would be great to fix them one by one to get a better understanding of the impact this PR could have on userland.

I'm already wondering if we shouldn't discuss about both directions of the conversion separately. It looks more of a smell to accept a bool where eg a string is expected, than to accept a string where a bool it required.

While I agree that coercing from bool to something else seem to always indicate a bug from doing fixes to the php-src tests, I have found a bunch of bugs in our test suite where strings are coerced to bool.
In our test suite the biggest causes for deprecation churn is using 0 and 1 for false and true respectively, and old and useless ZPP tests.

And the engine/type system simplification of true|false being the same as bool can't happen if coercing to bool is not deprecated :-/

This maps to structs like if ($boo) {...}. If this is accepted for any types (as it should), then the conversion rules are plain explicit and well known for the to-bool direction.

This is not exactly true, as values of type null, object, array, or resource do not pass a bool type check but are accepted by if ($value) {}

Places I found where the direction is from-bool are way less legit at first glance.

Can you try running the test suite? Here is the doc about how to: https://symfony.com/doc/current/contributing/code/tests.html

Then look at random deprecations and try to fix some to get some taste of the deprecation :)

I'll have a look at them soon-ish and will ask for help if I can't run the test suite.