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/**
 * 生成随机数
 *
 * @param int $length 生成字符串长度
 * @param int $type 字符串类型
 * @param bool $special 是否使用特殊字符
 * @return string 返回生成的随机字符串
 * @example random(10, null, true);
 */
function random($length, $type = NULL, $special = FALSE) {
	$str = "";
	switch ($type) {
		case 1:
			$str = "0123456789";
			break;
		case 2:
			$str = "abcdefghijklmnopqrstuvwxyz";
			break;
		case 3:
			$str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
			break;
		case 4:
			$str = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
			break;
		default:
			$str = "abcdefghijkmnpqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ23456789";
		break;
	}
	return substr(str_shuffle(($special != FALSE)  '!@#$%^&*()_+' . $str : $str), 0, $length);
}