Why is MD5'ing a UUID not a good idea?

A UUID is 128 bits wide and has uniqueness inherent to the way it is generated. A MD5 hash is 128 bits wide and doesn't guarantee uniquess, only a low probablity of collision. The MD5 hash is no smaller than the UUID so it doesn't help with storage.

If you know the hash is from a UUID it is much easier to attack because the domain of valid UUIDs is actually fairly predictable if you know anything about the machine geneerating them.

If you needed to provide a secure token then you would need to use a cryptographically secure random number generator.(1) UUIDs are not designed to be cryptographically secure, only guaranteed unique. A monotonically increasing sequence bounded by unique machine identifiers (typically a MAC) and time is still a perfectly valid UUID but highly predictable if you can reverse engineer a single UUID from the sequence of tokens.

1. The defining characteristic of a cryptographically secure PRNG is that the result of a given iteration does not contain enough information to infer the value of the next iteration - i.e. there is some hidden state in the generator that is not revealed in the number and cannot be inferred by examining a sequence of numbers from the PRNG.
   
   If you get into number theory you can find ways to guess the internal state of some PRNGs from a sequence of generated values. Mersenne Twister is an example of such a generator. It has hidden state that it used to get its long period but it is not cryptographically secure - you can take a fairly small sequence of numbers and use that to infer the internal state. Once you've done this you can use it to attack a cryptographic mechanism that depends on keeping that sequence a secret.

Note that uniqid() does not return a UUID, but a "unique" string based on the current time:

$ php -r 'echo uniqid("prefix_", true);'prefix_4a8aaada61b0f0.86531181

If you do that multiple times, you will get very similar output strings and everyone who is familiar with uniqid() will recognize the source algorithm. That way it is pretty easy to predict the next IDs that will be generated.

The advantage of md5()-ing the output, along with an application-specific salt string or random number, is a way harder to guess string:

$ php -r 'echo md5(uniqid("prefix_", true));'3dbb5221b203888fc0f41f5ef960f51b

Unlike plain uniqid(), this produces very different outputs every microsecond. Furthermore it does not reveil your "prefix salt" string, nor that you are using uniqid() under the hood. Without knowing the salt, it is very hard (consider it impossible) to guess the next ID.

In summary, I would disagree with the commentor's opinion and would always prefer the md5()-ed output over plain uniqid().

MD5ing a UUID is pointless because UUIDs are already unique and fixed length (short), properties that are some of the reasons that people often use MD5 to begin with. So I suppose it depends on what you plan on doing with the UUID, but in general a UUID has the same properties as some data that has been MD5'd, so why do both?