PHP 二进制数相加

<?php
    #二进制相加
    function binary_plus($binstr1, $binstr2) {
        $bin_arr1 = str_split($binstr1);
        $bin_arr2 = str_split($binstr2);
        $arr_len1 = count($bin_arr1);
        $arr_len2 = count($bin_arr2);
        $sum_arr = array();

        if ($arr_len1 < $arr_len2) {
            $short_arr = &$bin_arr1;
        } else {
            $short_arr = &$bin_arr2;
        }

        #将两个数组的长度补到一样长，短数组在前面补0
        for ($i = 0; $i < abs($arr_len1 - $arr_len2); $i++) {
            array_unshift($short_arr, 0);
        }

        $carry = 0;    #进位标记
        for ($i = count($bin_arr1) - 1; $i >= 0; $i--) { 
            $result = $bin_arr1[$i] + $bin_arr2[$i] + $carry;
            switch ($result) {
                case 0:
                    array_unshift($sum_arr, 0);
                    $carry = 0;
                    break;
                case 1:
                    array_unshift($sum_arr, 1);
                    $carry = 0;
                    break;
                case 2:
                    array_unshift($sum_arr, 0);
                    $carry = 1;
                    break;
                case 3:
                    array_unshift($sum_arr, 1);
                    $carry = 1;
                    break;
                default:
                    die();
            }
        }

        if($carry == 1) {
            array_unshift($sum_arr, 1);
        }

        return implode("", $sum_arr);
    }

    $bin1 = "1001101";
    $bin2 = "1010";
    print_r(binary_plus($bin1, $bin2));
?>