Description:
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The CAA record type is a security related DNS record type used to validate encryption  certificate issuers for a domain. In purpose it has quite a lot in common with HPKP certificate pinning, but with fewer downsides. Ideally it is combined with DNSSEC.

CAA is defined in RFC 6844: https://tools.ietf.org/html/rfc6844

It defines a new DNS record type with code 257 (see section 7.1 of the RFC)

PHP lacks support for this record type, so it cannot be queried using dns_get_record in PHP.

CAA is not yet seeing widespread take-up, however, it is supported by the letsncrypt.org CA, and that is becoming very popular - control panels etc that support DNS and TLS configuration are thus likely to need support for this record type in PHP.

Test script:
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var_dump(dns_get_record('php.net', DNS_CAA));