allow code (<?php echo $_GET['id']; ?>)

I hope there's a solution to my question.
How do allow the code (<?php echo $_GET['id']; ?>) to be posted in FCKeditor?
I have searched and tested, nothing works!

All suggestions attempted below don't work.
Removed ";"

FCKConfig.ProtectedSource.Add( /<\?[\s\S]*?\?>/g);