sorting a php array alphanumerically

I know there are natsort() and natcasesort() functions in php to sort array elements in natural order. I am trying to sort the following items.

array[0]= '10. xyz';

   array[1]=  '13. xyz';

   array[2]=  '2. xyz';

   array[3]=  '1a. xyz';

   array[4]=  '6. xyz';

   array[5]=  '1b. xyz';

   array[6]=  '4a. xyz';

   array[7]=  '4b. xyz';

now if I apply natsort(), '10..' and '13..' are placed above '1a..'. how can I make my php code to think that '10..' is greater than '1a...' and '1a...' should be at the top? the correct output in my particular case is:

'1a. xyz';

'1b. xyz';

'2. xyz';

'4a. xyz';

'4b. xyz';

'6. xyz';

'10. xyz';

'13. xyz';

i used a custom algo but it does the same...it places '10..' before '1a...'. heres my custom func:

function cmp($a, $b) {

    $a_ex=explode(".",$a);

    $b_ex=explode(".",$b);

    if ($a_ex[0] == $b_ex[0] ) {

        return 0;

    }

    return ($a_ex[0] < $b_ex[0]) ? -1 : 1;

}

Source: Tips4all