You can always decrypt it. We're talking about the encryption algorithms supported by mcrypt, not hashing algorithms like md5, sha1, crc32, etc. Those two things are for completely different purposes. If you weren't able to decrypt encrypted things, then how do you think things like PGP, GPG, TrueCrypt, BitLocker, FileVault, EFS, SSL etc. work?
I doubt it'd be easy to do. You could try to search for classes which does this.
You could take a look at this one which would take care of TEA, but PEAR doesn't have a class for Rijndael.