Not sure if what happened to me is the same as you, but when I first started on Zen Cart, I kept getting blank pages after modifying php files. Turns out, the copying and pasting I was doing (Notepad and Notepad++) was somehow dropping the line breaks and carriage returns (or supplanting them with the so-called unknown character that looks like a rectangle), so w/ code jammed up against the php command, the pages were breaking. It is still happening when I copy and paste w/ php files, but at least I know what's breaking the pages and can fix it.