"Empty delimiter" Warning when using PHP explode() function

I think str_split is what you are after

str_split — Convert a string to an array

<?php
$arr = str_split("Hello");
?>

will produce

Array
(
    [0] => H
    [1] => e
    [2] => l
    [3] => l
    [4] => o
}

If dealing with multi-byte UTF-8 strings you should use:

$array = preg_split('//u', $My_String,-1, PREG_SPLIT_NO_EMPTY);

Otherwise you can just use:

$array = str_split($My_String);

The reason is noted in the manual:

str_split() will split into bytes, rather than characters when dealing with a multi-byte encoded string.

Starting from PHP version 7.4 the mbstring equivalent of str_split was added so you can now use:

$array = mb_str_split($my_string);

mb_str_split manual page