/**
 +----------------------------------------------------------
 * 产生随机字串， 可用来自动生成密码，验证码，表单令牌等
 * 默认长度6位 字母和数字混合 支持中文
 +----------------------------------------------------------
 * @param string $len 长度
 * @param string $type 字串类型
 * 0 字母 1 数字 其它 混合
 * @param string $addChars 额外字符
 +----------------------------------------------------------
 * @return string
 +----------------------------------------------------------
 */
function rand_string($len = 6, $type = '', $addChars = '') {
	$str = '';
	switch ($type) {
		case 0 :
			$chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz' . $addChars;
			break;
		case 1 :
			$chars = str_repeat('0123456789', 3);
			break;
		case 2 :
			$chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' . $addChars;
			break;
		case 3 :
			$chars = 'abcdefghijklmnopqrstuvwxyz' . $addChars;
			break;
		default :
			// 默认去掉了容易混淆的字符oOLl和数字01，要添加请使用addChars参数
			$chars = 'ABCDEFGHIJKMNPQRSTUVWXYZabcdefghijkmnpqrstuvwxyz23456789' . $addChars;
			break;
	}
	if ($len > 10) { //位数过长重复字符串一定次数
		$chars = $type == 1 ? str_repeat($chars, $len) : str_repeat($chars, 5);
	}
	if ($type != 4) {
		$chars = str_shuffle($chars);
		$str = substr($chars, 0, $len);
	} else {
		// 中文随机字
		for ($i = 0; $i < $len; $i++) {
			$str .= msubstr($chars, floor(mt_rand(0, mb_strlen($chars, 'utf-8') - 1)), 1);
		}
	}
	return $str;
}