I'm interested in creating tiny url like links. My idea was to simply store an incrementing identifier for every long url posted and then convert this id to it's base 36 variant, like the following in PHP:
$tinyurl = base_convert($id, 10, 36)
The problem here is that the result is guessable, while it has to be hard to guess what the next url is going to be, while still being short (tiny). Eg. atm if my last tinyurl was a1, the next one will be a2. This is a bad thing for me.
So, how would I make sure that the resulting tiny url is not as guessable but still short?
What you are asking for is a balance between reduction of information (URLs to their indexes in your database), and artificial increase of information (to create holes in your sequence).
You have to decide how important both is for you. Another question is whether you just do not want sequential URLs to be guessable, or have them sufficiently random to make guessing any valid URL difficult.
Basically, you want to declare n out of N valid ids. Choose N smaller to make the URLs shorter, and make n smaller to generate URLs that are difficult to guess. Make n and N larger to generate more URLs when the shorter ones are taken.
To assign the ids, you can just take any kind of random generator or hash function and cap this to your target range N. If you detect a collision, choose the next random value. If you have reached a count of n unique ids, you must increase the range of your ID set (n and N).