key() technically returns internal array state:

<?php

$array = array(
  'a' => true,
  'z' => true,
);

var_dump(key($array));
next($array);
var_dump(key($array));

$ php -f test.php 
string(1) "a"
string(1) "z"

(I think until PHP7 foreach could interact with this, too.)

The chance that you'll ever get back a "funky" array is virtually nil, but you can use head_key() (or last_key()) from libphutil to avoid the risk.