I'm perfectly fine with the robustness principle being applied, but it seems as if it gives people who write poorly thought out projects a convenient excuse for bad design decisions. I've been using PHP since the early 4.x days, so I've seen the project change over the years and I can honestly say I don't think the robustness principle was ever consciously applied across the project, it just ended up that way. If it was a conscious decision then there would be at a very minimum a standard output for invalid input across all functions. Instead, every function returns something different (FALSE, 0, '', '0', NULL). Sometimes the output for invalid data is documented, sometimes it isn't, sometimes it changes without any notice or modification of the documentation. This doesn't seem like a design choice to me.