Description:
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The synopsis for flush() is "flush — Flush the output buffer". The description starts "Flushes the write buffers of PHP and whatever backend PHP is using (CGI, a web server, etc)."

This is quite misleading. The documentation latter explains:
" It also doesn't affect PHP's userspace output buffering mechanism. This means you will have to call both ob_flush() and flush() to flush the ob output buffers if you are using those. "

Therefore, there may be more than a single output buffer. I do not perfectly understand all buffers involved and I'm not sure about the terminology. What is the antonym of "userspace"? Quickly, I'd suggest:
flush — Flush system output buffers
"Flushes the system write buffer of PHP and, as much as possible, any write buffer of whatever backend PHP is using (CGI, a web server, etc)."

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From manual page: http://www.php.net/function.flush
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