Vale, lo he encontrado!.... Tenía un error en el área coloreada:
Cita: <?php
mysql_connect("localhost","usuarioDB","claveDB");
mysql_select_db("MiBD");
if(trim($HTTP_POST_VARS["accessname"]) != "" && trim($HTTP_POST_VARS["password"]) != "")
{
$accessname = $HTTP_POST_VARS["accessname"];
$password = $HTTP_POST_VARS["password"];
$result = mysql_query("SELECT password FROM usuarios WHERE accessname='$accessname'");
if($row = mysql_fetch_array($result))
{
if($row["password"] == $password)
{
?>
Adios...
POST RESUELTO!...