Detailed explanation of misunderstandings in using flush function

ob_flush/The description of flush in the manual is that they both refresh the output buffer and need to be used together, so it will cause confusion to many people... In fact, the two operations are different. In some cases , flush does nothing at all

ob_* series functions operate the output buffer of PHP itself.

So, ob_flush refreshes the buffer of PHP itself.

And flush, strictly speaking, this only has practical effect when PHP is installed as apache Module (handler or filter).
It refreshes the buffer of WebServer (can be considered specifically apache).

Under the sapi of apache module, flush will call the flush member of sapi_module Function pointer,
Indirectly call the api of apache: ap_rflush refreshes the output buffer of apache, of course, the manual also Having said that, there are some other modules of Apache,
may change the result of this action..

Some modules of Apache, such as mod_gzip, may perform output caching by themselves, which will cause the flush() function to be generated. The results are not sent to the client browser immediately.

Even the browser will cache the received content before displaying it. For example, the Netscape browser caches content until it receives a newline or the beginning of an html tag, and does not display the entire table until it receives the tag.

Some versions of Microsoft Internet Explorer
will only start displaying the page after receiving 256 bytes, so some additional spaces must be sent to allow these browsers to display the page content.

So, the correct order to use the two is. First ob_flush, then flush,

Of course, under other sapi, you can not call flush, just to ensure the portability of your code It is recommended to use it together.

<?php
// set_time_limit(0);
header(&#39;Content-Type: text/event-stream&#39;);
header(&#39;Cache-Control: no-cache&#39;);
// ob_end_flush();
// ini_set(&#39;output_buffering&#39;, 0);
// ini_set(&#39;implicit_flush&#39;, 1);
if (ob_get_level() == 0) ob_start();
echo str_repeat(&#39; &#39; ,4096);
$long = 60;
while($long > 0)
{
$time = date(&#39;r&#39;);
echo "data: The server time is: {$time}\n\n";
ob_flush();
flush();//break;
sleep(1);
$long --;
}
// var source=new EventSource("http://localhost:18000/sse.php");source.onmessage=function(event){console.info(event.data)};
?>

If you want to support nginx + fpm + php, you need to add a response header

header('X-Accel-Buffering: no');
This eliminates both proxy_buffering and (if you have nginx >= 1.5.6), fastcgi_buffering. The fastcgi bit is crucial if you're using php-fpm. The header is also far more convenient to do on an as-needed basis.
Docs on X-Accel-Buffering http://nginx.org/en/docs/http/ngx_http_fastcgi_module.html#fastcgi_buffering ;

<?php
// set_time_limit(0);
header(&#39;Content-Type: text/event-stream&#39;);
header(&#39;Cache-Control: no-cache&#39;);
header(&#39;X-Accel-Buffering: no&#39;);

// ob_end_flush();
// ini_set(&#39;output_buffering&#39;, 0);
// ini_set(&#39;implicit_flush&#39;, 1);

// if (ob_get_level() == 0) ob_start();
// echo str_repeat(&#39; &#39; ,4096);

$long = 60;
while($long > 0)
{
	$time = date(&#39;r&#39;);
	echo "data: The server time is: {$time}\n\n";
	ob_flush();
	
	flush();//break;
	sleep(1);
	$long --;
}
// var source=new EventSource("http://localhost:18000/sse.php");source.onmessage=function(event){console.info(event.data)};
?>

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