Re: Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in

[Paul McArdle]

This is a very general error. Most of the time it just means that you could not connect to the database (for whatever reason) and thus the result set you tried to fetch was not a "valid MySQL result resource".
You should try first to get a more descriptive error message

if ($row = mysql_fetch_row($res)) {
return $row;
} else {
print (mysql_error());
}