How Can I Generate an Email List from a MySQL Table

I have a MySQL table named "users" in which there is a column named "email_address" and another named "is_subscribed". I would like to send an email to all of the addresses for which "is_subscribed" is true. I think I know how to send the email, i.e., mail($to, $subject, $message, $headers); I tested this out and it works fine when, for instance, $to = "recipient1@abc.com, recipient2@xyz.com" or some such collection of addresses. My problem is how do you create the variable, $to, from the MySQL table so that it contains all of the addresses separated by commas. Thanks for your help, Roger Lewis