echo DNS_CAA, DNS_A, DNS_CNAME, DNS_HINFO, DNS_MX, DNS_NS, 
     DNS_PTR, DNS_SOA, DNS_TXT, DNS_AAAA, DNS_SRV, DNS_NAPTR,
     DNS_A6, DNS_ALL, DNS_ANY;

If the above line is put into PHP/syntax_test_php.php, the first item, DNS_CAA is highlighted differently to the rest of the line. According to the dns_get_record#type parameter list, it is a valid constant.