Does PHP's flush() do anything if running from terminal/cli? - php

Question

I'm starting php directly from the terminal and passing it a .php script to run, I'm not using a web browser.
The documentation for flush() says
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.
So based on that, it sounds possible that flush() could still be needed when you need output to immediately get sent to the terminal before the script ends, because of the part where it said "system write buffers of PHP". But I would like to know definitively. I'm using php 8.1 currently, but my question is hopefully not version specific.
I'm wanting to know from someone familiar with PHP's C implementation whether it ever under any circumstances buffers output when started from a terminal, or if that only happens if PHP is started a special way by a server. I know if I run code that intentionally requests buffering by using ob_start() or something like that, then it is supposed to buffer and this will not be ended by calling flush(), that's not what I'm asking about. I'm asking about code that doesn't explicitly request any buffering.

Answer 1

Why not test that yourself?
<?php
// ob_start();
print "Test 1\n";
sleep(1);
print "Test 2";
sleep(5);
print "Test 3";
flush();
sleep(5);
On my test terminal (standard Ubuntu 20.04.5 LTS), ob_start buffers everything, and ignores flush() altogether. Without ob_start, all text is output immediately (even if not ended by a LF, as is the case for the second test).
I haven't a PHP 8.1 handy, but mine says,
> PHP 8.2.1 (cli) (built: Jan 18 2023 10:17:15) (NTS)
> Copyright (c) The PHP Group
> Zend Engine v4.2.1, Copyright (c) Zend Technologies
You might ask a different (opposite, even) question: are there conditions whereby my terminal-run script might start buffering for some reason?
When you call flush() the call gets ultimately redirected to sapi_module, which issues a fflush() on stdout (this on the current source tree on Github, but I don't think this was changed since at least PHP 5). This is enough, I believe, to say that flush() is sufficient.
Is it necessary? On a standard run from the terminal, no. But I didn't include any scriptlets or libraries or Composer modules that might have changed the buffering strategy.
Of course, any of those could have. Just issue an ob_start() (maybe because the module is born to run on a web server) and hey presto!, your script starts buffering and requires flush (actually, in that case, ob_flush() - as said above, flush will not work).
But if that's the case, you've got a completely different beast to deal with - you should probably write your own output function, so that all console writes are routed to some code that can check, say, ob_get_level() and deal with it accordingly. Or, bringing this to the extreme,
declare(ticks=1);
register_tick_function(static function() {
ob_get_level() && ob_flush();
flush();
});