PHP将字符串转换为数组

$str = 'abcdef';

我想得到：

array(6) {
  [0]=>
  string(1) "a"
  [1]=>
  string(1) "b"
  [2]=>
  string(1) "c"
  [3]=>
  string(1) "d"
  [4]=>
  string(1) "e"
  [5]=>
  string(1) "f"
}