flash-construction header error message

Hi,

My site got hacked and upon restoring, the following message showed up at the header section.

Warning: Use of undefined constant HTTP_USER_AGENT – assumed ‘HTTP_USER_AGENT’ (this will throw an Error in a future version of PHP) in /home/yyy/xxxx.com/wp content/themes/flash/header.php on line 1

How to resolve this?

Regards,
Meijer D.