Imploding an array?

["Aaron Wolski" <aaronjw@martekbiz.com>]

Hey all,
 
Can't seem to figure out why this won't work:
 
$test = implode(",", $order_index[$i]);
 
When I print_r() the variable I have values so I know that works.
 
Any thoughts on this?
 
Thanks.
 
Aaron