private function = Call to undefined function error

[eluser]stef25[/eluser]
ive got some processing code in my controller id like to break out in to a separate function.

the function in question is at the bottom and its called from

Code:

$existing_tags = array_flatten($existing_tags);

Code:

function array_flatten($array)
    {
        $ab = array(); if(!is_array($a)) return $ab;
        foreach($a as $value){
            if(is_array($value)){
                $ab = array_merge($ab,array_flatten($value));
            }else{
                array_push($ab,$value);
            }
        }
        return $ab;
    }

this returns a "Call to undefined function array_flatten()" even though both snippets are in the same controller file.

i dont want this function to be accessible from the url but i removed the _underscore just to debug.

this is basic php but im stumped. anyone?