Using hook to replace header banner to 728×90

I’m trying to use a hook to replace the top header banner which is 468×60 with a leaderboard banner call full_top.

This is the hook which I’ve placed at the very top in my child themes functions.php file but I get a “Can’t redeclare…” error.

function hook_banner_header($c){ return $CORE->BANNER(‘full_top’) apply_filters(‘hook_banner_header’, $c); }

What am I doing wrong?