PHP buffer ob_flush() vs. flush()

Question

What's the difference between ob_flush() and flush() and why must I call both?

The ob_flush() reference says:

This function will send the contents of the output buffer (if any).

The flush() reference says:

Flushes the write buffers of PHP and whatever backend PHP is using (CGI, a web server, etc).

However, it continues to say:

[it] may not be able to override the buffering scheme of your web server…

So, seems to me that I could just use ob_flush() all of the time. However, I get strange results when I do that. Could someone explain in simple terms what's going on here?

Answer 1

ob_flush sends an application-initiated buffer. There may be multiple nested ob_start()'s in any PHP script. ob_flush passes the current content to the upper layer.

PHP itself might (at its own discretion) buffer output. This depends on the back-end. But usually FastCGI has a socket buffer on its own. Therefore flush() needs to be invoked as well to send the current content to the web server.

And now the web server might itself implement another buffering scheme (mod_deflate or content filter), which you have no influence over. But this is seldom, as it needs to be configured specifically.

Anyway, use both.