That is 16 bytes of binary data encoded with base64, which is about right for an MD5 hash.
Hashes are one-way functions, so you cannot recover the input.
If you want to convert the hash to a more familiar hexadecimal representation:
$foo = "RTSi6+yxLwIW7KsnoehVDA==";
$hex = bin2hex(base64_decode($foo));
var_dump($hex); // output: string(32) "4534a2ebecb12f0216ecab27a1e8550c"