高精度函数实现

/**
 * 十进制数转换成62进制
 *
 * @param integer $num
 * @return string
 */
function from10_to62($num) {
    $to = 62;
    $dict = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $ret = '';
    do {
        $ret = $dict[bcmod($num, $to)] . $ret;
        $num = bcdiv($num, $to);
    } while ($num > 0);
    return $ret;
}

/**
 * 62进制数转换成十进制数
 *
 * @param string $num
 * @return string
 */
function from62_to10($num) {
    $from = 62;
    $num = strval($num);
    $dict = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $len = strlen($num);
    $dec = 0;
    for($i = 0; $i < $len; $i++) {
        $pos = strpos($dict, $num[$i]);
        $dec = bcadd(bcmul(bcpow($from, $len - $i - 1), $pos), $dec);
    }
    return $dec;
}

普通版

/**
 * 10进制转为62进制
 *
 * @param integer $n 10进制数值
 * @return string 62进制
 */
function dec62($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;
}

/**
 * 62进制转为10进制
 *
 * @param integer $n 62进制
 * @return string 10进制
 */
function dec10($s) {
    $base = 62;
    $index = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $ret = 0;
    $len = strlen($s) - 1;
    for($t = 0; $t <= $len; $t ++) {
        $ret += strpos($index, substr($s, $t, 1)) * pow($base, $len - $t);
    }
    return $ret;
}