<?php
function mbStrSplit ($string, $len=1) {
  $start = 0;
  $strlen = mb_strlen($string);
  while ($strlen) {
    $array[] = mb_substr($string,$start,$len,"utf8");
    $string = mb_substr($string, $len, $strlen,"utf8");
    $strlen = mb_strlen($string);
  }
  return $array;
}

header('Content-type:text/html;charset=utf-8');
$str = '我愛北京3我愛上海-我愛xianggang';
$r = mbStrSplit($str, 4);
echo '<pre>';
print_r($r);
echo '</pre>';
?>

Array
(
  [0] => 我愛北京
  [1] => 3我愛上
  [2] => 海-我愛
  [3] => xian
  [4] => ggan
  [5] => g
)

<?php
 function str_split_unicode($str, $l = 0) {
 if ($l > 0) {
 $ret = array();
 $len = mb_strlen($str,"UTF-8");
 for ($i = 0; $i < $len; $i += $l) {
 $ret[] = mb_substr($str, $i, $l,"UTF-8");
}
 return $ret;
}
 return preg_split("http://u", $str, -1, PREG_SPLIT_NO_EMPTY);
}
?>
 $s ="還未如愿sss見gfg者不rtret不朽"; // Mild milk
 print_r(str_split($s, 5));
 print_r(str_split_unicode($s, 5));

//輸出
 //Array ( [0] => 還�� [1] => �如� [2] => ��sss [3] => 見gf [4] => g者� [5] => ��rtr [6] => et不 [7] => 朽 )
 //Array ( [0] => 還未如愿s [1] => ss見gf [2] => g者不rt [3] => ret不朽 )