Printing an element of an Array?

[The Hobo]

If I define an array like this:

$arr = array(a => 'apple', b=> 'banana');

How can I access the first element of the array without knowing that the key is a?

echo $arr[?];

Would normally do $arr['a'], but let's say I don't know that the first element's key is a.

Thanks in advance.