I have a design I’m working on that is almost complete, but I’m having trouble getting the CSS to do what I want. What I am trying to do is something like this:

However, when the page is generated, this is what the output looks like:

How can I get the third box to float up under the first and next to the second? I have tried every trick I know, but can’t get it to work. I should also mention that each block is added to the page by a loop in PHP pulling from a database, so I’m kinda limited by not having static content, and have no way of knowing ahead of time how tall a particular block is going to be.
If the number of columns is variable, CSS can’t really do it (if you want it to work in all common browsers), so instead use jQuery Masonry which is designed to solve exactly this problem.
Here’s me saying the same thing, but with more words: CSS two columns with known children width