Keep Getting Error: php_network_getaddresses: getaddrinfo failed

Hi - I am using this url_exists function: to check whether files exist
on other peoples webservers (before allowing my users to add the site
to their directory of rss feeds). On my current shared hosting (running
Apache 1.3.29 Red Hat Linux PHP 4.3.10) the function works perfectly.
On my new, Plesk VPS system (running Apache 2.0.51 Fedora PHP 4.3.10)
it doesn't work - I get this error:

php_network_getaddresses: getaddrinfo failed

If the is the IP address then the error does not occur but if I try to
resolve the IP address using gethostbyname("webaddress") it simply
returnes the webaddress (e.g. bbc.co.uk returns bbc.co.uk - not the IP
address).

It looks like the issue relates to the DNS server hence putting the
query here in apache configuration - apologies if it turns out to be a
specific PHP problem. The DNS Server is BIND.

Many thanks.

Here is the function:

function url_exists($url) {
$a_url = parse_url($url);
if (!isset($a_url['port'])) $a_url['port'] = 80;
$errno = 0;
$errstr = '';
$timeout = 5;
if(isset($a_url['host']) &&
$a_url['host']!=gethostbyname($a_url['host'])){
$fid = fsockopen($a_url['host'], $a_url['port'], $errno,
$errstr, $timeout);
if (!$fid) return false;
$page = isset($a_url['path']) ?$a_url['path']:'';
$page .= isset($a_url['query'])?'?'.$a_url['query']:'';
fputs($fid, 'HEAD '.$page.' HTTP/1.0'."\r\n".'Host:
'.$a_url['host']."\r\n\r\n");
$head = fread($fid, 4096);
fclose($fid);
return preg_match('#^HTTP/.*\s+[200|302]+\s#i', $head);
} else {
return false;
}
}