Simple short Encrypt string to fixed length n matter what the input string length is

The input string could be of varying length but the encrypted string must be a max of 15 characters and alphanumeric.

This is clearly impossible. If you solved this, you would solve all possible storage concerns - after all, you'd be able to store the whole internet (viewed as one long string) in 15 alphanumeric characters (after decryption).

You haven't told us what the input string might consist of, either - assuming that by "alphanumeric" you mean A-Z, a-z, 0-9 that's only 62 possible characters. So there are 62¹⁵ possible encrypted strings. If your input is in UTF-16 code units, then just 6 code units has more possibilities (65536⁶ is greater than 62¹⁵).

Basically, you're onto a losing proposition here - you should rethink your design.

Perhaps you should store the original value in a server, return some token, and then be able to fetch the value when you want? That isn't encryption, but it may satisfy your real requirements.

Further reading: the pigeonhole principle