global variables

[eluser]Uplift[/eluser]
I am new to code igniter and cant work something out.

Basically, in the header i have an ads section that i want to on every page load.

for this i am using an array

$ads = array(
"ad1 code...",
"ad2 code...",
"ad3 code...",
);

then returning it like this

$data['advertisements'] = random_element($ads);

the problem is the ads appear on every page and having this code in every controller will be a nightmare to update. how can i declare it globally?