mb_split

a (simpler) way to extract all characters from a UTF-8 string to array with a single call to a built-in function:
<?php
 $str = 'Ма-
руся';
 print_r(preg_split('//u', $str, null, PREG_SPLIT_NO_EMPTY));
?>

Output:
Array
(
 [0] => М
 [1] => а
 [2] => -
 [3] => 
 [4] => р
 [5] => у
 [6] => с
 [7] => я
)

The $pattern argument doesn't use /pattern/ delimiters, unlike other regex functions such as preg_match.
<?php
 # Works. No slashes around the /pattern/
 print_r( mb_split("\s", "hello world") );
 Array (
 [0] => hello
 [1] => world
 )
 # Doesn't work:
 print_r( mb_split("/\s/", "hello world") );
 Array (
 [0] => hello world
 )
?>

I figure most people will want a simple way to break-up a multibyte string into its individual characters. Here's a function I'm using to do that. Change UTF-8 to your chosen encoding method.
<?php
function mbStringToArray ($string) {
 $strlen = mb_strlen($string);
 while ($strlen) {
 $array[] = mb_substr($string,0,1,"UTF-8");
 $string = mb_substr($string,1,$strlen,"UTF-8");
 $strlen = mb_strlen($string);
 }
 return $array;
}
?>

In addition to Sezer Yalcin's tip.
This function splits a multibyte string into an array of characters. Comparable to str_split().
<?php
function mb_str_split( $string ) {
 # Split at all position not after the start: ^
 # and not before the end: $
 return preg_split('/(?<!^)(?!$)/u', $string );
}
$string = '火车票';
$charlist = mb_str_split( $string );
print_r( $charlist );
?>

# Prints:
Array
(
 [0] => 火
 [1] => 车
 [2] => 票
)

To split an string like this: "日、に、本、ほん、語、ご" using the "、" delimiter i used:
 $v = mb_split('、',"日、に、本、ほん、語、ご");
but didn't work.
The solution was to set this before:
 mb_regex_encoding('UTF-8');
 mb_internal_encoding("UTF-8"); 
 $v = mb_split('、',"日、に、本、ほん、語、ご");
and now it's working:
Array
(
 [0] => 日
 [1] => に
 [2] => 本
 [3] => ほん
 [4] => 語
 [5] => ご
)

I agree that some people might want a mb_explode('', $string);
this is my solution for it:
<?php
$string = 'Hallöle';
$array = array_map(function ($i) use ($string) { 
 return mb_substr($string, $i, 1); 
}, range(0, mb_strlen($string) -1));
expect($array)->toEqual(['H', 'a', 'l', 'l', 'ö', 'l', 'e']);
?>

We are talking about Multi Byte ( e.g. UTF-8) strings here, so preg_split will fail for the following string: 
'Weiße Rosen sind nicht grün!'
And because I didn't find a regex to simulate a str_split I optimized the first solution from adjwilli a bit:
<?php
$string = 'Weiße Rosen sind nicht grün!'
$stop = mb_strlen( $string);
$result = array();
for( $idx = 0; $idx < $stop; $idx++)
{
 $result[] = mb_substr( $string, $idx, 1);
}
?>

Here is an example with adjwilli's function:
<?php
mb_internal_encoding( 'UTF-8');
mb_regex_encoding( 'UTF-8'); 
function mbStringToArray
( $string
)
{
 $stop = mb_strlen( $string);
 $result = array();
 for( $idx = 0; $idx < $stop; $idx++)
 {
 $result[] = mb_substr( $string, $idx, 1);
 }
 return $result;
}
echo '<pre>', PHP_EOL, 
print_r( mbStringToArray( 'Weiße Rosen sind nicht grün!', true)), PHP_EOL,
'</pre>'; 
?>

Let me know [by personal email], if someone found a regex to simulate a str_split with mb_split.

an other way to str_split multibyte string:
<?php
$s='әӘөүҗңһ';
//$temp_s=iconv('UTF-8','UTF-16',$s);
$temp_s=mb_convert_encoding($s,'UTF-16','UTF-8');
$temp_a=str_split($temp_s,4);
$temp_a_len=count($temp_a);
for($i=0;$i<$temp_a_len;$i++){
 //$temp_a[$i]=iconv('UTF-16','UTF-8',$temp_a[$i]);
 $temp_a[$i]=mb_convert_encoding($temp_a[$i],'UTF-8','UTF-16');
}
echo('<pre>');
print_r($temp_a);
echo('</pre>');
//also possible to directly use UTF-16:
define('SLS',mb_convert_encoding('/','UTF-16'));
$temp_s=mb_convert_encoding($s,'UTF-16','UTF-8');
$temp_a=str_split($temp_s,4);
$temp_s=implode(SLS,$temp_a);
$temp_s=mb_convert_encoding($temp_s,'UTF-8','UTF-16');
echo($temp_s);
?>