Checking Multiple Checkboxes At Once?

I have a list of names with checkboxes in front of them. The list is generated from a database. So in a loop it would look something like this:

<input type=checkbox value='$userID' name='recipients[]'> $UserName

The name of the checkbox is a Array, because I want to pass the information on to another page, where the emails-addresses are used to send a mail.

the userID is also linked with the email-account of the user. What I want to do is to have a link, button or checkbox (which one is the easiest to implement) that checks all the checkboxes in the list at once. How do I go about this?

I have come up with the following, but this doesn't seem to work: PHP Code:

function CheckEm(cb_mail){
  for (var i = 0; i < document.myForm[cb_mail].length; i++)
    document.myForm[cb_mail][i].checked = true;
}

<input type=checkbox onClick="CheckEm('cb_mail')">