PHP custom interception of Chinese string-utf8 version

First of all, please note: There are currently a lot of codes for this problem on the Internet, but many of them are copied and pasted without my own practice, and the code has logical problems. The following code is written by myself.

Not much to say

/**
   * 该函数是对于utf8编码
   * @author 2582308253@qq.com
   * @param string $str
   * @param int $start
   * @param int $length
   * @return string
   * @copyright 2017年2月27日下午1:46:10
   */
  function gbsubstr2($str, $start, $length) {
    $length = abs($length);
    $strLen = strlen($str);
    $len = $start + $length;
    $newStr = '';
    for($i = $start; $i < $len && $i < $strLen; $i++) {
      if(ord(substr($str, $i, 1)) > 0xa0) {
        //utf8编码中一个汉字是占据3个字节的，对于其他的编码的字符串，中文占据的字节各有不同，自己需要去修改这个数a
        $newStr .= substr($str, $i, 3);//此处a=3;
        $i+=2;
        $len += 2; //截取了三个字节之后，截取字符串的终止偏移量也要随着每次汉字的截取增加a-1;
      } else {
        $newStr .= substr($str, $i, 1);
      }
    }
    return $newStr;
  }

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