If you can't see obvious patterns, that does not make it cryptographically secure! See also my answer to this question on our sibling site: What does it mean for a random number generator to be cryptographically secure?
At the risk of stating the obvious, rand() and mt_rand() are NOT SECURE for security purposes. Do NOT use them for things like shuffling a ...
That look like base64. If the strings content [A-Z,a-z,0-9,+,/,=] then you can straight apply the following command:
echo "your strings" | base64 -d
Otherwise visit sites which can do this for you eg: https://www.base64decode.org/
I generally visit two websites to decode/decrypt i.e. https://www.dcode.fr/ and https://gchq.github.io/CyberChef/.