Adding disallow file edits problem

Hi all

I am trying to add the:
define('DISALLOW_FILE_EDIT', true);
string to my wp-config.php but whenever and wherever I place it I recieve a php error of
unexpected T_STRING in wp-config.php on line 1 which is the:
<?php tag!
I am certain I am not removing or “breaking” any comments causing something else to be commented out etc. and am sure nothing else is being changed.

Any help would be greatly appreciated
Aventura