[php-src] Issue #7894: filter_var("false", FILTER_VALIDATE_BOOL returns the set default value

Issue: https://github.com/php/php-src/issues/7894 Comment Author: damianwadley Oh, I didn't notice that either. Technically this behavior is correct: without FILTER_NULL_ON_FAILURE, the filter only "allows" truthy values, and since "false" is not truthy it's rejected - and the default is used. That flag adds an additional behavior to give special handling to falsey values, so no flag means no special handling. It might be a bit surprising, but (a) it's documented like this, (b) there is a "workaround" of providing FILTER_NULL_ON_FAILURE, and (c) changing this could break compatibility.