How to add `$a[null]` without `uasort`?

my code is

```
$a = array(3=>'fff',4=>'sss');
end($a);
$key = key($a);
for($i=0;$i<=($key);$i++){
    if(!isset($a[$i])){$a[$i]=null;}
}

```

**i want this** 

`([0] => null [1] => null ...... [3] => fff [4] => sss )`

thank you