int uniqid(string prefix);
uniqid() returns a prefixed unique identifier based on current time in microseconds. The prefix can be useful for instance if you generate identifiers simultaneously on several hosts that might happen to generate the identifier at the same microsecond. The prefix can be up to 114 characters long.
If you need a unique identifier or token and you intend to give out that token to the user via the network (i.e. session cookies), it is recommended that you use something along the lines of
$token = md5(uniqid("")); // no random portion $better_token = md5(uniqid(rand())); // better, difficult to guess
This will create a 32 character identifier (a 128 bit hex number) that is extremely difficult to predict.