Description:
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At least for Q encoding, this function is unsafe and does not encode correctly. Raw characters which appear as RFC2047 sequences are simply left as is.

Ex:

mb_encode_mimeheader( '=?iso-8859-1?q?this=20is=20some=20text?=' );

returns '=?iso-8859-1?q?this=20is=20some=20text?='

The exact same string, which is obviously not the encoding for the source string.  That is, mb_encode_mimeheader does not do any type of escaping.

That is, the following condition is not always true:
    mb_decode_mimeheader( mb_encode_mimeheader( $text ) ) == $text

Reproduce code:
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$text = '=?iso-8859-1?q?this=20is=20some=20text?=';

assert( mb_decode_mimeheader( mb_encode_mimeheader( $text ) ) == $text );

Expected result:
----------------
The decode/encode sequence should always return the original text.


Actual result:
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Returned result is different than original (that is, the assertion fails).