Description:
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PHP supports the IPV6_RECVPKTINFO constant with sockets extension.
However, on macOS, it is not appeared.

On macOS, in.h is exported the constant only when __APPLE_USE_RFC_3542 is defined:
https://opensource.apple.com/source/xnu/xnu-4570.41.2/bsd/netinet6/in6.h.auto.html

Test script:
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<?php

var_dump(defined(IPV6_RECVPKTINFO));
var_dump(IPV6_RECVPKTINFO);

Expected result:
----------------
bool(true)
int(61)

Actual result:
--------------
bool(false)
string(16) "IPV6_RECVPKTINFO"