Description:
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Please give htmlentities an option to encode all codes that are not ASCII even if numerical encoding is needed.

Maybe the option could be a "accepted base encoding" like:

string htmlentities  ( string $string  [, int $quote_style  [, string $charset  [, bool $double_encode  [, string $base_charset  ]]]] )

Reproduce code:
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Having a generated page that needs to stay in latin-1 encoding because it is part of a bigger project, you want to include some dynamic content in cyrilic (e.g. from user input encoded in UTF-8).

Expected result:
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encode all non-ASCII and non-character-entity-characters as something like Ӓ

Actual result:
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they left untouched which gives encoding errors