After searching SO and other sites, I've failed to come up with conclusive evidence to how Facebook, Twitter and Pinterest generate their ID's. The reason this is needed is to avoid url collisions. Moving to an entirely different ID will prevent this because there wont be quadrillions of records.
If you look at Pinterest as an example, the first few digits relate to the user id, and the last 6 or so digits represent the save id which possibly could be an auto increment.
To create a similar ID, but not unique I was able to use: base_convert(user_id.save_id, 16, 10). The problem here is that it's not unique, ex: base_convert(15.211, 16, 10) vs. base_convert(152.11, 16, 10). These two are the same. Simply just merging two unique sets of numbers will still produce duplicate results. Throwing uniqid() into the mix will essentially fix the duplicates, but this doesn't seem like a great practice.
Update: Twitter appears to use this: https://github.com/twitter/snowflake
Any suggestions on generating a unique ID like the above examples?
The Flickr comment up above was very useful. We use sharding as well. We have an bigint (int64) locator field. It is generated by combining an int (int32) database id and an int (int32) identity field.
If you know you will have an int16 number of database max (quite likely), you could combine an int16 (smallint) database id and an int32 (int) user id and an int16 (smallint) action id. I don't know reasonable numbers for your application. But reserve some part for the database id, even if it's just tinyint, so you know you're future safe if you add more databases.
Actually, if you look at (for example) the IDs of users on your Friends (on Facebook), you'd notice that they are sequential among all users, exactly like an AUTO_INCREMENT database field. However, they probably don't start at 1. My friends list, for example, has some numbers in the millions, then suddenly jump to 1 trillion and something, so my guess is that the auto_increment value was bumped up - this may be done to "hide" exactly how many users there are.
Anyway, to generate unique IDs, just create them sequentially with that AUTO_INCREMENT field. Optionally, set the initial value to something high.
Suppose your IDs are all numeric. Delimit them by a character A (since it surely does not appear in the original IDs) and do a base conversion from base-11 to base-10.
For the example you did we now get different results:
echo base_convert("15A211", 11, 10); //247820
echo base_convert("152A11", 11, 10); //238140