php如何把一个20位的62进制的转回10进制字符串

Question

php如何把一个20位的62进制的转回10进制字符串。
目前通过

    function dec62($n) {
        $base = 62;
        $index = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $ret = '';
        for($t = floor(log10($n) / log10($base)); $t >= 0; $t --) {
            $a = floor($n / pow($base, $t));
            $ret .= substr($index, $a, 1);
            $n -= $a * pow($base, $t);
        }
        return $ret;
    }

可以把超长的十进制整数转到62进制，但是由于系统限制，转回来的时候会变成9.9999999991447E+27这样的数字。
需要一个算法，把超长的62进制转回10进制字符串。
可以用下面这个数字测试
9999999999144705880199999999999

Answer 1

请使用 BCMath 做任意精度的加减乘除。

function base62to10($n)
{
    $index = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $n = strval($n);
    $len = strlen($n);
    $result = 0;
    $base = 1;
    for ($i = $len-1; $i >= 0; $i--) {
        $char = $n[$i];
        $d = strpos($index, $char);
        assert($d !== false);
        $result = bcadd($result, bcmul($d, $base));
        $base = bcmul($base, 62);
    }
    return $result;
}

echo base62to10('9999999999144705880199999999999'),"\n";

Answer 2

首先，楼主的算法是不对的，因为我用9999999999144705880199999999999和9999999999144705880199999999998这个数测dec62(),发现输出结果是一样的。
我在php手册中看到了大神的算法(需要BCMath支持)：

    function convBase($numberInput, $fromBaseInput, $toBaseInput){
        if ($fromBaseInput==$toBaseInput) return $numberInput;
        $fromBase = str_split($fromBaseInput,1);
        $toBase = str_split($toBaseInput,1);
        $number = str_split($numberInput,1);
        $fromLen=strlen($fromBaseInput);
        $toLen=strlen($toBaseInput);
        $numberLen=strlen($numberInput);
        $retval='';
        if ($toBaseInput == '0123456789')
        {
            $retval=0;
            for ($i = 1;$i <= $numberLen; $i++)
                $retval = bcadd($retval, bcmul(array_search($number[$i-1], $fromBase),bcpow($fromLen,$numberLen-$i)));
            return $retval;
        }
        if ($fromBaseInput != '0123456789')
            $base10=convBase($numberInput, $fromBaseInput, '0123456789');
        else
            $base10 = $numberInput;
        if ($base10<strlen($toBaseInput))
            return $toBase[$base10];
        while($base10 != '0')
        {
            $retval = $toBase[bcmod($base10,$toLen)].$retval;
            $base10 = bcdiv($base10,$toLen,0);
        }
        return $retval;
    }

用法：
1、十进制转62进制

echo convBase('9999999999144705880199999999999','0123456789','0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ');

2、62进制转十进制

echo convBase('3nLqycbr6ZQsN1JJYX','0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ','0123456789');

它支持任意形式的进制转换，详见 http://php.net/manual/en/function.base-convert.php