PHP. Руководство по PHP. 2005
uniqid
uniqid
Description
string uniqid ([string prefix [, bool more_entropy]])uniqid() returns a prefixed unique identifier based on the current time in microseconds. prefix is optional but can be useful, for instance, if you generate identifiers simultaneously on several hosts that might happen to generate the identifier at the same microsecond. Up until PHP 4.3.1, prefix could only be a maximum of 114 characters long.
If the optional more_entropy parameter is TRUE, uniqid() will add additional entropy (using the combined linear congruential generator) at the end of the return value, which should make the results more unique.
With an empty prefix, the returned string will be 13 characters long. If more_entropy is TRUE, it will be 23 characters.
Замечание: The prefix parameter became optional in PHP 5.