I have two arrays:

```
$a = array([0]=>1 [1]=>2 [2]=>3);
$b = array([0]=>a [1]=>b [2]=>c);

```

I want to merge them like

```
$ab=array( [a]=>array([0]=>1 [1]=>2 [2]=>3) 
           [b]=>array([0]=>a [1]=>b [2]=>c)  );

```

How to do this ?

I tried `array_merge()` but it does not work as I want it to.