问PHP: gethostbyname()和checkdnsrr()的结果不一致

EN

$sdomains = array();
array_push($sdomains, 'etc.com');

function domainAvailable1($domain){
$results = @dns_get_record($domain, DNS_ANY);
return empty($results);
}
function domainAvailable2($domain){
return gethostbyname($domain) == $domain;
}
function domainAvailable3($domain){
return !checkdnsrr($domain, 'ANY');
}

echo '<table><tr><td>@dns_get_record</td><td>gethostbyname</td><td>checkdnsrr</td></tr>';       

foreach($sdomains as $sdomain){
if (domainAvailable1($sdomain) == true){                    
            echo '<tr><td bgcolor=green>' . $sdomain . ' is available!</td>';
            } else {
            echo '<tr><td bgcolor=red>' . $sdomain . ' is NOT available.</td>';
            }
if (domainAvailable2($sdomain) == true){                    
            echo '<td bgcolor=green>' . $sdomain . ' is available!</td>';
            } else {
            echo '<td bgcolor=red>' . $sdomain . ' is NOT available.</td>';
            }
if (domainAvailable3($sdomain) == true){                    
            echo '<td bgcolor=green>' . $sdomain . ' is available!</td></tr>';
            } else {
            echo '<td bgcolor=red>' . $sdomain . ' is NOT available.</td></tr>';
            }
}