Is a timestamp in microseconds always unique?

Microsecond based ids are only guaranteed to be unique within limits. A single threaded scripts on a single computer is probably pretty safe in this regard. However, as soon as you start talking about parallel execution, be that simply on multiple CPUs within the same machine or especially across multiple machines, all bets are off.

So it depends on what you want to use this id for. If you're just using it to generate an id which is used only within the same script, it's probably safe enough. For example:

<?php $randomId = uniqid(); ?><div id="<?php echo $randomId; ?>"></div><script>    var div = document.getElementById('<?php echo $randomId; ?>');    ...</script>

You very likely won't encounter any problems here with this limited use.

However, if you start generating file names using uniqid or other such uses which are shared with other external scripts, I wouldn't rely on it. For filenames, using a hash based on the file contents may be a good idea. For general purpose decentralised randomly generated ids, UUIDs are a good fit (because they've been designed for this purpose).

Ask yourself why you need uniqid in the first place. For instance, I use uniquid as the filename of uploads to my website. There can be any number of users who upload at the same time so what I am concerned with is two or more files having the same id, BUT I know that a single user can only upload one file at a time. So, I prepend the username in front and will always have uniqueness.

echo uniqid('username-'); // username-5621e3335ac0c

Of course, you should always ask yourself if you need to use uniquid in the first place. If you know the reason you are creating the id can only happen every x seconds, minutes, etc then you can create an id the same way just use time :

echo 'username-'.time(); // username-1445062025