General idea is to use curl. Here is some example:
<?php
// create a new CURL resource
$ch = curl_init();
// set URL and other appropriate options
curl_setopt($ch, CURLOPT_URL, "http://www.example.com/");
curl_setopt($ch, CURLOPT_HEADER, false);
// grab URL and pass it to the browser
curl_exec($ch);
// close CURL resource, and free up system resources
curl_close($ch);
?>
You will just need to use curl_setopt function to add POST or GET parameters. Read the manual for details