根据id转 邀请码

/**
 * @description: 10进制转36进制
 * @param {type} 
 * @return: 
 */
public function createCode($userId,$sum)
{

    static $sourceString = [
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
        'a', 'b', 'c', 'd', 'e', 'f',
        'g', 'h', 'i', 'j', 'k', 'l',
        'm', 'n', 'o', 'p', 'q', 'r',
        's', 't', 'u', 'v', 'w', 'x',
        'y', 'z'
    ];
 
    $num = $userId;
    $code = '';
 
    while ($num) {
        //精确求余/取模
        $mod = bcmod($num, 36);
        //精确除法
        $num = bcdiv($num, 36);
        $code = $sourceString[$mod].$code; //邀请码拼接
    }

    //判断code的长度
    if (!isset($code[$sum]))
        $code=str_pad($code, $sum, '0', STR_PAD_LEFT); //长度不够拼接'0'

    return $code;
}