Description:
------------
Consider: 

<?php echo gethostbyname("www.google.com"); ?>
38.7.248.176

<?php echo gethostbyname("downloads.wordpress.org"); ?>
0.0.0.0

;; ANSWER SECTION:
www.google.com.		229	IN	A	216.58.193.196

;; ANSWER SECTION:
downloads.wordpress.org. 300	IN	A	198.143.164.250

The DNS on the host is running fine, DNS for downloads.wordpress.org is ok. We are however running on a dual-stack ipv4/v6 host. 

www.google.com has an AAAA record, downloads.wordpress.org does not. 

I suspect that gethostbyname() is returning an AAAA record from the system call to gethostbyname(3), which returns a struct hostent: 

           struct hostent {
               char  *h_name;            /* official name of host */
               char **h_aliases;         /* alias list */
               int    h_addrtype;        /* host address type */
               int    h_length;          /* length of address */
               char **h_addr_list;       /* list of addresses */
           }

While the PHP docs explicitly state that PHP's gethostbyname() returns the IPv4 address, the implementation does not restrict or check if the system call actually returns an AF_INET h_addrtype instead of AF_INET6, always assuming a v4 and copying sizeof(in_addr): https://github.com/php/php-src/blob/master/ext/standard/dns.c#L272 

According to the Linux man pages, "The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror() functions are obsolete.  Applications should use getaddrinfo(3), getnameinfo(3), and gai_strerror(3) instead."

"The  gethostbyaddr()  function  returns  a  structure  of  type  hostent  for the given host address addr of length len and address type type.  Valid address types are AF_INET and
       AF_INET6."

 
This problem has manifested elsewhere in this Wordpress bug report from a dual-stack host: https://core.trac.wordpress.org/ticket/38291