PHP（MySQL）错误：“警告：mysql_num_rows（）期望参数1为资源” ?mysql

if (!empty($_POST)){

$email_to=$_POST['email_to'];
$email_to=mysql_real_escape_string($_POST['email_to']);

$sql = "UPDATE `cosmos`.`members` SET `conf` = '2' WHERE `members`.`email` = '$email_to';";
$result=mysql_query($sql) or trigger_error(mysql_error().$sql);

$count=mysql_affected_rows($result);                  // line 20
if($count==1){

$rows=mysql_fetch_array($result);
$unique=$rows['u_code'];
$name=$rows['username'];
// ---------------- SEND MAIL FORM ---------------- 
$to=$email_to; 
$subject="Your Account Password Request! - Cosmos"; 
$header="from: Tayal's/Cosmos <cosmos@gmail.com>"; 
$messages= "Hey $name ,\r\n";
$messages.="You recently requested a new password";
$messages.="<br /><a href='confirm.php?uid" . $unique . "'>Confirmation Link</a> \r\n";
$sentmail = mail($to,$subject,$messages,$header); 
echo $messages; 
}   else {
echo "Not found your email in our database";
}

} 警告：mysql_affected_rows（）期望参数1为资源，在第20行的C：\ wamp \ www \ a \ l \ forget.php中给出布尔值