Effettuare una login tramite script php invece che tramite un form POST

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sì, con le socket non è difficile.

ad esempio questa funzione serve per inviare un trackback, ma fare un loging è la stessa cosa:

function send_ping($tb_url, $pb_url, $pb_title='', $pb_excerpt='', $pb_blog_name='', $charset='utf-8'){
  $url = parse_url($tb_url);
  $host = $url['host'];
  $pb_id = $url['path'] ."?" .$url['query'];
  $post_data = "url=".urlencode($pb_url)
             ."&title=".urlencode($pb_title)
             ."&excerpt=".urlencode($pb_excerpt)
             ."&blog_name=".urlencode($pb_blog_name);

  $sk = fsockopen($host, 80, $errno, $errstr, 30);
  if(!$sk) echo "Errore $errno: $errstr";
  else{
    $headers = "POST $pb_id HTTP/1.0\r\n";
    $headers .= "Host: $host\r\n";
    $headers .= "User-Agent: LIT TrackBack lib\r\n";
    $headers .= "Content-Type: application/x-www-form-urlencoded; charset=$charset\r\n";
    $headers .= "Content-Length: ".strlen($post_data)."\r\n\r\n";

    fputs($sk, $headers.$post_data);
    while (!feof($sk)):
          $resp .= fgets($sk);
    endwhile;
    fclose($sk);
    return $resp;
  }
}