Why can I not change the "flagged" property for imap email?

[king.oslo]

Hello,

 

I am using this code to try and unflag imap messages, it works for flags such as "Seen" but not the "Flagged" property. Any idea why?:

 

<?php
$status = imap_clearflag_full($connection, $msgNumber, '\\Flagged');
?>

 

Thanks,

Marius