Description:
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In source code https://github.com/php/php-src/blob/master/ext/intl/idn/idn.c#L159 I see that comparison with 255 bytes is after covertation to Unicode by uidna_nameToUnicodeUTF8() function and it is wrong.
So domain 'абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаеж.рф' has 264 bytes, so idn_to_utf8() function returns false.



RFC 589: https://tools.ietf.org/rfc/rfc5890#page-16. U-label Lengths section:
"Because A-labels (the form actually used in the DNS) are potentially much more compressed than UTF-8 (and UTF-8 is, in general, more compressed that UTF-16 or UTF-32), U-labels that obey all of the relevant symmetry (and other) constraints of these documents may be quite a bit longer, potentially up to 252 characters (Unicode code points).  A fully-qualified domain name containing several such labels can obviously also exceed the nominal 255 octet limit for such names.  Application authors using U-labels must exert due caution to avoid buffer overflow and truncation errors and attacks in contexts where shorter strings are expected."

So, it is need compare with 255 bytes punycode form of domain, not Unicode.


Test script:
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$punycode = idn_to_ascii('абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаеж.рф', IDNA_DEFAULT, INTL_IDNA_VARIANT_UTS46);

$unicode = idn_to_utf8($punycode, IDNA_DEFAULT, INTL_IDNA_VARIANT_UTS46);

var_dump($unicode);



Expected result:
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string(294) "абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаежзи.абвгдаеж.рф"

Actual result:
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bool(false)