Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

Sorry, haven't read carefully UPGRADING

 - pack()/unpack() had the following changes, which bring it more in line
  with Perl's behavior:
  - Implemented format character "Z": NUL-padded string
  - "a" now does not remove trailing NUL characters on unpack() anymore
  - "A" will now strip all trailing ASCII whitespace on unpack() (it used to
    remove only trailing spaces).

This was intentional — specifically, it was to fix bug #61038. Copy-pasting from the migration guide (http://php.net/manual/en/migration55.changed-functions.php):

"unpack()'s a format no longer strips trailing null bytes, bringing it into line with Perl's behaviour."

Snap. :)

There still a problem with new 'z' format:

php -r '$a="\x00\x00\x00\x00"; var_dump($a); var_dump(unpack("Z4foo", $a));'

Actual result:
string(4) "^@^@^@^@"
array(1) {
  ["foo"]=>
  string(1) "^@"
}

When we expect:
string(4) "^@^@^@^@"
array(1) {
  ["foo"]=>
  string(0) ""
}