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);
?>