Update, March 2014:
Firstly, it is important to note that
uniqid is a bit of a misnomer as it doesnt guarantee a unique ID.
Per the PHP documentation:
This function does not create random nor unpredictable string. This function must not be used for security purposes. Use cryptographically secure random function/generator and cryptographically secure hash functions to create unpredictable secure ID.
This function does not generate cryptographically secure tokens, in fact without being passed any additional parameters the return value is little different from microtime(). If you need to generate cryptographically secure tokens use openssl_random_pseudo_bytes().
Setting more-entropy to true generates a more unique value, however the execution time is longer (though to a tiny degree), according to the docs:
If set to TRUE, uniqid() will add additional entropy (using the combined linear congruential generator) at the end of the return value, which increases the likelihood that the result will be unique.
Note the line
increases the likelihood that the result will be unique and not that is will guarantee uniqueness.
You can 'endlessly' strive for uniqueness, up to a point, and enhance using any number of encryption routines, adding salts and the like- it depends on the purpose.
I'd recommend looking at the comments on the main PHP topic, notably:
What I'd recommend is working out why you need uniqueness, is it for security (i.e. to add to an encryption/scrambling routine)? Also, How unique does it need to be? Finally, look at the speed consideration. Suitability will change with the underlying considerations.