PHP - uniqid(“”,true) versus uniqid(“”)+mt_rand()

Question

What are the key differences between these two approaches to generate sequential-but-somewhat-unique numbers? I want to use such a number as a unique user ID inside a MySQL db,

Answer 1

(scratch this, wrong info, sorry)

So yes, as Gumbo pointed out, it's not a hash but indeed a microsecond resolution increasing timer value, optionally concatenated with a random number. So, the values are still not sequential, but at least steady. The difference between the two approaches would then be very small.

This one still holds: If you need unique sequential numbers, and you use MySQL, use an auto increment field, they do just that.