You can use
$first_key = key($array);
It's essentially the same as your initial code, but with a little less overhead, and it's more obvious what is happening.
Just remember to call
reset, or you may get any of the keys in the array. You can also use
end instead of
reset to get the last key.
If you wanted the key to get the first value,
reset actually returns it:
$first_value = reset($array);
There is one special case to watch out for though (so check the length of the array first):
$arr1 = array(false);
$arr2 = array();
var_dump(reset($arr1) === reset($arr2)); // bool(true)