php生成短链接：将数字转成字母，将字母转成数字

一、基类方法：定义字符串、编写转换方法
class NumberHelper
{
    public static $str="abcdefghijklmnopqrstuvwxyz6789ABCDEFGHIJKLMNOPQRSTUVWXYZ012345";

/**
 * Param:将数字转为短字符串
 * @param $number
 * @return string
 */
public static function generate_code($number) {
    $out  = "";
    $codes = self::$str;
    while ($number > 61) {
        $m = $number % 62;
        $out = $codes[$m].$out;
        $number = ($number - $m) / 62;
    }
    return $codes[$number].$out;
}

/**
 * Param:将短字符串转为数字
 * @param $string
 * @return float|int
 */
public static function get_num($string){
    $codes = self::$str;
    $num = 0;
    for($i=0;$i<strlen($string);$i++){
        $n = strlen($string) - $i -1;
        $pos = strpos($codes,$string[$i]);
        $num += $pos * pow(62, $n);
    }
    return $num;
}

}

二、调用转换方法，根据自己定的规则，生成自己需要的短链接

public static function getSh