PHP靠谱的随机数加字母生成

/**
 * 随机生成数字字母组合
 *
 * @param int  $len 长度
 * @param bool $small_letter 小写
 * @return string
 */
function getRandomString($len = 0, $small_letter = FALSE)
{
	if ( $small_letter )
	{
		$chars = "abcdefghijklmnopqrstuvwxyz0123456789";
	}
	else
	{
		$chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
	}
	mt_srand(10000000 * (double)microtime());
	for ($i = 0, $str = '', $lc = strlen($chars) - 1; $i < $len; $i++)
	{
		$str .= $chars[ mt_rand(0, $lc) ];
	}

	return $str;
}