一个能生成各种模式随机字符串的函数

/** *随机字符串生成函数 * @param int $length 生成长度 * @param int $mode 字符模式 * @return string  */function generate($length=5, $mode) {    $str = $gen_str = '';    switch ($mode) {        case 1:            $str = '0123456789';            break;        case 2:            $str = 'abcdefghijklmnopqrstuvwxyz';            break;        case 3:            $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';            break;        case 4:            $str = 'abcdefghijklmnopqrstuvwxyz0123456789';            break;        case 5:            $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';            break;        case 6:            $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';            break;        default:            $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';            break;    }    for ($i = 0; $i < $length; $i++) {        $num = mt_rand(0, strlen($str) - 1);        $gen_str .= $str[$num];    }    return $gen_str;}