Try using curl (either the command line version using system() or exec()) or use the curl extension. Look under www.php.net/manual/en/ for the curl functions.
If you don't have curl built into your PHP, download curl and compile it on your system. Then use system() or exec() to call curl.
In case you are not familiar with curl: it's a program to fetch remote URLs. It has the ability to POST data (meaning pass in name=value pairs using POST).
-sridhar