Shoutbox

well I recomend using curl to do it because its super simple.

code:

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$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_HEADER, 0); //dont put the header recived into the data
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1); //to not echo the data back
curl_setopt($ch, CURLOPT_POST, 1); // set POST method
curl_setopt($ch, CURLOPT_POSTFIELDS, 'blah=xxxx&moo=444'); //post string.
$foo = curl_exec($ch); //foo is now filled with the page it returned.
curl_close($ch);

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for further reading on curl http://php.net/curl

well do you know what data you need to send?
if you dont download live http header extention
goto the site you want to view the headers for  click tools , Live Http Headers
then a window will appear and you should be able to figure out how to read the POST from there.