php man page for flush

FLUSH(3)								 1								  FLUSH(3)

flush - Flush system output buffer

SYNOPSIS
       void flush (void  )

DESCRIPTION
	Flushes  the  system write buffers of PHP and whatever backend PHP is using (CGI, a web server, etc). This attempts to push current output
       all the way to the browser with a few caveats.

       flush(3) may not be able to override the buffering scheme of your web server and it has no effect  on  any  client-side	buffering  in  the
       browser.  It also doesn't affect PHP's userspace output buffering mechanism. This means you will have to call both ob_flush(3) and flush(3)
       to flush the ob output buffers if you are using those.

	Several servers, especially on Win32, will still buffer the output from your script until it terminates before transmitting the results to
       the browser.

	Server	modules  for  Apache like mod_gzip may do buffering of their own that will cause flush(3) to not result in data being sent immedi-
       ately to the client.

	Even the browser may buffer its input before displaying it. Netscape, for example, buffers text until it receives an  end-of-line  or  the
       beginning of a tag, and it won't render tables until the </table> tag of the outermost table is seen.

	Some  versions of Microsoft Internet Explorer will only start to display the page after they have received 256 bytes of output, so you may
       need to send extra whitespace before flushing to get those browsers to display the page.

RETURN VALUES
	No value is returned.

SEE ALSO
       ob_flush(3), ob_clean(3), ob_end_flush(3), ob_end_clean(3).

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