So I was debugging some anomalous behavior on my website and as I was poking around, I half-consciously noticed the change bars in in my editor that showed two extra newlines at the end of a .php file. But I dismissed this even as soon as I saw it because… I mean, it's just whitespace. At the end of a file.
Later I noticed a warning in the logs about “Cannot Modify Header Information”. And it was just a warning, but I didn't remember seeing it before, so I did some googling. And what I read reminded me that the way PHP works is that it tries to emit anything outside <?php ?> flags back to the web browser.
“What the heck” I thought, and removed the newlines. Immediately the site started to work again.
So lesson learned: watch out for whitespace in .php files.
This also made something else I'd seen in PHP-land make more sense. I'd noticed that some authors leave the closing ?> off of the end of files that contain only PHP code and no HTML. This offended the well-formedness centers of my brain, and I'd previously written it off as laziness. But now I understand that this is idiomatic, and for good reasons.
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