how is the other form submitted? via post?
something like:
<?
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,"http://www.mysite.com/tester.phtml");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, "postvar1=value1&postvar2=value2&postvar3=value3");
curl_exec ($ch);
curl_close ($ch);
?>