explode string by letter in php

I have a string in php like this one:

$string = "Count the character frequency of this string";

When I used explode function the output display as:

Array ( 
        [0] => Count 
        [1] => the 
        [2] => character 
        [3] => frequency 
        [4] => of 
        [5] => this 
        [6] => string 
       ) 

But the output above doesn't satisfied me because I want to achieve an output which looks like this:

Array ( 
    [0] => C 
    [1] => o 
    [2] => u 
    [3] => n 
    [4] => t 
    [5] => t 
    [6] => h 
    [7] => e 
    [8] => c 
    [9] => h 
    [10] => a 
    [11] => r 
    [12] => a 
    [13] => c 
   )

I want to explode them by letter.

My question: Is it possible to do that? If it is, any suggestions. Thanks in advance