OK, figured out this bizarreness. My big mistake: assuming a white screen of death was some kind of code error with PHP. It was actually some badly coded css on my part that hid the entire body from view! All I had to do is look at the source code to see all the code was there.
So that explains why there were no errors and why throwing an echo statement that output to the screen magically “fixed” things; because it broke the css rendering.
One of those days.
And pro tip: don’t work on 3 hours of sleep.