Mail all users in my database

So im trying to make a script so i can mail all users in my database mailing them about updates etc the problem i have i can send a email to one user but not muti would this work?

  $sql = 'SELECT email  FROM users ORDER BY id';
    foreach ($db->query($sql) as $row) {

  $to = $_row['email'];
$subject =  $_POST['subject'];
$txt = $_POST['comment'];
$headers = "From: admin@myemail.com" . "" .
"CC: admin@myemail.com";

mail($to,$subject,$txt,$headers);

    }

I off course set the posts in a form before this is posted but would this be fine ?