Max lenght of a variable is usually around 900 ($len() wise).
So you could simply limit it to 900, and if it reaches 900 ull add a message that its full or so. OR!
You can use a hash table (Dont know what the limit is, but obviously easier to deal with when it comes to several ID\'s).
If none of thats good enough, you can use dyanmic variables, which you\'ll loop twice.

First you add the items in variable, if it reaches 900, u use variable.1, and so on, and add the id\'s in another variable
so in the end u might have

%variable.1
%variable.2
%variable.3

%variable.all 3

That way you\'ll know, \"aha, there\'s 3 dyanmic vars\" and then echo the info.

Ill let u figure it out urself ;)