Currently imap_mutf7_to_utf8 doesn't support U+10000 and up.
You can test this with the teapot emoji (PHP 5 >= 5.3.0, PHP 7, PHP 8)
imap_mutf7_to_utf8 — Decode a modified UTF-7 string to UTF-8
$string): string|falseDecode a modified UTF-7 (as specified in RFC 2060, section 5.1.3) string to UTF-8.
注意:
This function is only available, if libcclient exports utf8_to_mutf7().
stringA string encoded in modified UTF-7.
Returns string converted to UTF-8,
或者在失败时返回 false.
Currently imap_mutf7_to_utf8 doesn't support U+10000 and up.
You can test this with the teapot emoji