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(Sorry if this is 'too early' and will be addressed prior to release anyway)
At present, https://github.com/php/php-src/blob/php-8.1.0RC1/NEWS and https://github.com/php/php-src/blob/php-8.1.0RC1/UPGRADING mention the new 'readonly properties' feature under new features, but don't mention under 'Backward Incompatible Changes' that readonly can't be used as a class/function name/etc.
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Thanks. There is a PR to change this, so I think it's a good idea to wait with documenting the exact behaviour. The UPGRADING file can still be changed if it contains missing information. The documentation pages haven't been created yet to my knowledge.
FTR, the PR has been applied.