Simon Willison’s Weblog

How can I get access to the PHP script of websites like Dropbox?

If a website doesn’t deliberately publish its server-side code (some sites like reddit do this, but it’s pretty rare) then you won’t be able to see it. You can search for an open source clone but these will often be pretty low quality—the smartest open source developers tend to work on libraries that solve common problems rather than putting their efforts in to building complete clones of existing sites.