REPLACE THESE LINES

$result = mysql_query("SELECT * from matthes_berliner_r_accounts WHERE user='$username' and password='$password'");
$count = mysql_num_rows($result);

WITH

$sql = "SELECT * from matthes_berliner_r_accounts WHERE user='$username' and password='$password'";
$result = mysql_query($sql);
if (!$result)
{
echo mysql_error();
}