public static function replaceStar($str, $start, $length = 0)
{
  $i = 0;
  $star = '';
  if($start >= 0) {
   if($length > 0) {
    $str_len = strlen($str);
    $count = $length;
    if($start >= $str_len) {//当开始的下标大于字符串长度的时候，就不做替换了
     $count = 0;
    }
   }elseif($length < 0){
    $str_len = strlen($str);
    $count = abs($length);
    if($start >= $str_len) {//当开始的下标大于字符串长度的时候，由于是反向的，就从最后那个字符的下标开始
     $start = $str_len - 1;
    }
    $offset = $start - $count + 1;//起点下标减去数量，计算偏移量
    $count = $offset >= 0 ? abs($length) : ($start + 1);//偏移量大于等于0说明没有超过最左边，小于0了说明超过了最左边，就用起点到最左边的长度
    $start = $offset >= 0 ? $offset : 0;//从最左边或左边的某个位置开始
   }else {
    $str_len = strlen($str);
    $count = $str_len - $start;//计算要替换的数量
   }
  }else {
   if($length > 0) {
    $offset = abs($start);
    $count = $offset >= $length ? $length : $offset;//大于等于长度的时候 没有超出最右边
   }elseif($length < 0){
    $str_len = strlen($str);
    $end = $str_len + $start;//计算偏移的结尾值
    $offset = abs($start + $length) - 1;//计算偏移量，由于都是负数就加起来
    $start = $str_len - $offset;//计算起点值
    $start = $start >= 0 ? $start : 0;
    $count = $end - $start + 1;
   }else {
    $str_len = strlen($str);
    $count = $str_len + $start + 1;//计算需要偏移的长度
    $start = 0;
   }
  }

  while ($i < $count) {
   $star .= '*';
   $i++;
  }

  return substr_replace($str, $star, $start, $count);
}

public function testReplaceStar()
 {
  $actual = App_Util_String::replaceStar('123456789', 3, 2);
  $this->assertEquals($actual, '123**6789');
  
  $actual = App_Util_String::replaceStar('123456789', 9);
  $this->assertEquals($actual, '123456789');
  
  $actual = App_Util_String::replaceStar('123456789', 9, 2);
  $this->assertEquals($actual, '123456789');
  
  $actual = App_Util_String::replaceStar('123456789', 9, -9);
  $this->assertEquals($actual, '*********');
  
  $actual = App_Util_String::replaceStar('123456789', 9, -10);
  $this->assertEquals($actual, '*********');
  
  $actual = App_Util_String::replaceStar('123456789', 9, -11);
  $this->assertEquals($actual, '*********');
  
  $actual = App_Util_String::replaceStar('123456789', 3);
  $this->assertEquals($actual, '123******');
  
  $actual = App_Util_String::replaceStar('123456789', 0);
  $this->assertEquals($actual, '*********');
  
  $actual = App_Util_String::replaceStar('123456789', 0, 2);
  $this->assertEquals($actual, '**3456789');

  $actual = App_Util_String::replaceStar('123456789', 3, -3);
  $this->assertEquals($actual, '1***56789');
  
  $actual = App_Util_String::replaceStar('123456789', 1, -5);
  $this->assertEquals($actual, '**3456789');
  
  $actual = App_Util_String::replaceStar('123456789', 3, -3);
  $this->assertEquals($actual, '1***56789');
  
  $actual = App_Util_String::replaceStar('123456789', -3, 2);
  $this->assertEquals($actual, '123456**9');
  
  $actual = App_Util_String::replaceStar('123456789', -3, 5);
  $this->assertEquals($actual, '123456***');
  
  $actual = App_Util_String::replaceStar('123456789', -1, 2);
  $this->assertEquals($actual, '12345678*');
  
  $actual = App_Util_String::replaceStar('123456789', -1, -2);
  $this->assertEquals($actual, '1234567**');
  
  $actual = App_Util_String::replaceStar('123456789', -4, -7);
  $this->assertEquals($actual, '******789');
  
  $actual = App_Util_String::replaceStar('123456789', -1, -3);
  $this->assertEquals($actual, '123456***');
  
  $actual = App_Util_String::replaceStar('123456789', -1);
  $this->assertEquals($actual, '*********');
  
  $actual = App_Util_String::replaceStar('123456789', -2);
  $this->assertEquals($actual, '********9');
  
  $actual = App_Util_String::replaceStar('123456789', -9);
  $this->assertEquals($actual, '*23456789');
  
  $actual = App_Util_String::replaceStar('123456789', -10);
  $this->assertEquals($actual, '123456789');
  
  $actual = App_Util_String::replaceStar('123456789', -10, -2);
  $this->assertEquals($actual, '123456789');
 }