You need to supply user agents with your requests.

Read: http://lists.wikimedia.org/pipermail/wikitech-l/2010-February/046777.html

Try: ini_set( 'user_agent', 'Erel\'s bot' );

Thank you, it works only servers where ini_set is enabled.

I found a solution that works on all servers, including shared hosting, where ini_set is usually disabled:

function get_url_with_agent($url) {

$ch = curl_init($url);
curl_setopt($ch, CURLOPT_HTTPGET, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_USERAGENT, 'Erel Bot');
$result = curl_exec($ch);
curl_close($ch);
return $result;

}

Hope it helps someone.