php函数将字符串分成不同的字符串？

$str = "Hello, World!";
$arr = str_split($str);
print_r($arr);

Array
(
    [0] => H
    [1] => e
    [2] => l
    [3] => l
    [4] => o
    [5] => ,
    [6] =>  
    [7] => W
    [8] => o
    [9] => r
    [10] => l
    [11] => d
    [12] => !
)

$str = "Hello, World!";
$arr = preg_split("//u", $str, -1, PREG_SPLIT_NO_EMPTY);
print_r($arr);

Array
(
    [0] => H
    [1] => e
    [2] => l
    [3] => l
    [4] => o
    [5] => ,
    [6] =>  
    [7] => W
    [8] => o
    [9] => r
    [10] => l
    [11] => d
    [12] => !
)