variable as default parameter in function?

[eluser]gscharlemann[/eluser]
I'm reading some data from a config file into an array. Is there a way to do this (I think php is barfing on my default parameter assignment)

Code:

class Tracker_model extends Model
{
    public $tables = array();

    public $schedule_sort_by = array();


    public function __construct()
    {
        parent::Model();
        $this->load->config('tracker');
        $this->tables = $this->config->item('tables');
        $this->schedule_sort_by = $this->config->item('schedule_sort_by');
    }

    public function get_user_schedule($user_id, $sort_by = $schedule_sort_by['scheduled_date'])
    {
          ...
    }
}

$schedule_sort_by['scheduled_date] is the "default" column heading I would like to sort by if it's not specified. I could hard code it, but the column name might change based on setup, that's why it's defined in the config file.

thanks!