Description:
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[RFC2047 section 4.2](https://tools.ietf.org/html/rfc2047#section-4.2) describes a way of encoding 8-bit characters sets in email headers, and in PHP that's handled by the mbstring extension. In that section, spaces can be encoded using either `=20` or `_`, the latter being preferable as it is more readable and uses fewer characters. A header encoded this way might look like this:

    X-My-Header: =?us-ascii?Q?hello_world?=

(this is a simplistic example - that header value does not actually *need* RFC2047 encoding, though it is harmless)

The mb_decode_mimeheader function does not decode this correctly, leaving the underscore undecoded. It does decode the alternative `=20` syntax correctly.

A workaround is to encode the `_` as `=20` prior to decoding, as in:

    mb_decode_mimeheader(str_replace('_', '=20', 'X-My-Header: =?us-ascii?Q?hello_world?='))

Note that this should not be applied blindly because the header may not be Q-encoded in the first place.

Test script:
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echo mb_decode_mimeheader('X-My-Header: =?us-ascii?Q?hello_world?=');


Expected result:
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X-My-Header: hello world

Actual result:
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X-My-Header: hello_world