Ciao
Ti posto questo codice che ho rimediato su php.net e provato personalmente..
Sono 2 funzioni, GET e POST, diciamo che ciò che devi settare è la variabile $query e host
codice:
$useragent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 4.0)";

function post($host,$query,$others='') { Global $useragent;
   $path=explode('/',$host);
   $host=$path[0];
   unset($path[0]);
   $path='/'.(implode('/',$path));
   $post="POST $path HTTP/1.1\r\nHost: $host\r\nContent-type: application/x-www-form-urlencoded\r\n{$others}User-Agent: $useragent\r\nContent-length: ".strlen($query)."\r\nConnection: close\r\n\r\n$query";
   $h=fsockopen($host,80);
   fwrite($h,$post);
   for($a=0,$r='';!$a;){
       $b=fread($h,8192);
       $r.=$b;
       $a=(($b=='')?1:0);
   }
   fclose($h);
   return $r;
}
function get($host,$query,$others='') { Global $useragent;
   $path=explode('/',$host);
   $host=$path[0];
   unset($path[0]);
   $path='/'.(implode('/',$path));
   $post="GET $path HTTP/1.1\r\nHost: $host\r\nContent-type: application/x-www-form-urlencoded\r\n{$others}User-Agent: $useragent\r\nContent-length: ".strlen($query)."\r\nConnection: close\r\n\r\n$query";
   $h=fsockopen($host,80);
   fwrite($h,$post);
   for($a=0,$r='';!$a;){
       $b=fread($h,8192);
       $r.=$b;
       $a=(($b=='')?1:0);
   }
   fclose($h);
   return $r;
}

//ESEMPIO
$host = "www.sito.com";
$query = "var=54&prova=si";
$altro = "";

echo $test = post($host,$query,$altro);