This one needs more explanations.

raises E_DEPRECATE. Use empty value to use default_charset setting and/or
use input_encoding, output_encoding, internal_encoding.
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raises E_DEPRECATED. "default_charset" is used as the default. Set empty values for "mbstring.http_input", "mbstring.http_output", "mbstring.internal_encoding" to use global "default_charset"/"input_encoding"/"output_encoding"/"internal_encoding" setting. Unlike iconv, mstring input/output INI does not use "default_charset". Input/output conversion is not needed when "default_charset" and "mbstring.internal_encoding" is the same.