Keep getting: php_network_getaddresses: getaddrinfo failed

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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. returns - not the IP

It looks like the issue relates to the DNS server so apologies if this
is a server question not a PHP question - but as the issue is
manifesting itself only in my PHP I thought I'd ask here too.

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']) &&
           $fid = fsockopen($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:
           $head = fread($fid, 4096);
           return preg_match('#^HTTP/.*\s+[200|302]+\s#i', $head);
       } else {
           return false;

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