PHPが文字列をインターセプトしたときに中国語が文字化けする問題の解決策

mb_substr('截取中文乱码问题测试',0,5, 'utf-8');

string substr (string string, int start [, int length])
$rest = substr ("abcdef", 1); // returns "bcdef"
$rest = substr ("abcdef", 1, 3); // returns "bcd"

$rest = substr ("abcdef", -1); // returns "f"
$rest = substr ("abcdef", -2); // returns "ef"
$rest = substr ("abcdef", -3, 1); // returns "d"

$rest = substr ("abcdef", 1, -1); // returns "bcde"

$rest = substr ("中国人", 1, -1); // returns "fdsafsda" 就是乱码了

echo mb_substr("php中文字符encode",0,4,"utf-8");

function sub_str($str, $length = , $append = true)
{
  $str = trim($str);
  $strlength = strlen($str);
  if ($length == || $length >= $strlength)
  {
    return $str; //截取长度等于或大于等于本字符串的长度，返回字符串本身
  }
  elseif ($length < ) //如果截取长度为负数
  {
    $length = $strlength + $length;//那么截取长度就等于字符串长度减去截取长度
    if ($length < )
    {
      $length = $strlength;//如果截取长度的绝对值大于字符串本身长度，则截取长度取字符串本身的长度
    }
  }
  if (function_exists(&#39;mb_substr&#39;))
  {
    $newstr = mb_substr($str, , $length, EC_CHARSET);
  }
  elseif (function_exists(&#39;iconv_substr&#39;))
  {
    $newstr = iconv_substr($str, , $length, EC_CHARSET);
  }
  else
  {
    //$newstr = trim_right(substr($str, , $length));
    $newstr = substr($str, , $length);
  }
  if ($append && $str != $newstr)
  {
    $newstr .= &#39;...&#39;;
  }
  return $newstr;
}