Get host domain name function

I found it through a search. I want to use it and post it to see if there are any problems!
echo $HTTP_HOST
This is the domain name to be exported!

function PMA_getenv($var_name) {
 if (isset ( $_SERVER [$var_name] )) {
 return $_SERVER [$var_name];
 } elseif (isset ( $_ENV [$var_name] )) {
 return $_ENV [$var_name];
 } elseif (getenv ( $var_name )) {
 return getenv ( $var_name );
 } elseif (function_exists ( 'apache_getenv' ) && apache_getenv ( $var_name, true )) {
 return apache_getenv ( $var_name, true );
 }
 return '';
}
if (empty ( $HTTP_HOST )) {
 if (PMA_getenv ( 'HTTP_HOST' )) {
 $HTTP_HOST = PMA_getenv ( 'HTTP_HOST' );
 } else {
 $HTTP_HOST = '';
 }
}
echo htmlspecialchars ( $HTTP_HOST );