$len = mb_strlen("你是我的小苹果","utf-8");

$str = "你是我的小苹果";

php > $str = "你是我的小苹果";
php > echo $str{0};
�

php > echo $str{0}.$str{1}.$str{2};

<?php

function mbstrlen($str,$encoding="utf8")
{

  if (($len = strlen($str)) == 0) {
    return 0;
  }

  $encoding = strtolower($encoding);

  if ($encoding == "utf8" or $encoding == "utf-8") {
    $step = 3;
  } elseif ($encoding == "gbk" or $encoding == "gb2312") {
    $step = 2;
  } else {
    return false;
  }

  $count = 0;
  for ($i=0; $i<$len; $i++) {
    $count++;
    //如果字节码大于127，则根据编码跳几个字节
    if (ord($str{$i}) >= 0x80) {
      $i = $i + $step - 1;//之所以减去1，因为for循环本身还要$i++
    }
  }
  return $count;
}

echo mbstrlen(iconv("utf-8","gbk","你是我的小苹果"),"gbk");
echo mbstrlen("你是我的小苹果");