php 10进制转62进制，可用于短网址生成

<?php

/**
 * 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;
}