Description

It's an historical bug !!
The following code:

<?php
echo 'Hello';
ini_set('session.name', 'SID');

Resulted in this output:

Hello
Warning: ini_set(): Session ini settings cannot be changed after headers have already been sent in /in/HIReN on line 4

But I expected this output instead:

Hello

For a lot of time the developers by pass this problem with tricks:
https://3v4l.org/rJdZo

<?php
fwrite(STDOUT, 'Hello');
ini_set('session.name', 'SID');
fwrite(STDOUT, "\n" . ini_get('session.name'));

And also setting an error handler, because when output an error happen the same:
https://3v4l.org/3lEcj

<?php
set_error_handler(function ($code, $msg, $file, $line) {
 fwrite(\STDERR, "$msg in file $file on line $line\n");
});
$foo = $bar;
ini_set('session.name', 'SID');
echo ini_get('session.name'), PHP_EOL;

The first difference of the CLI SAPI is that no headers are written to the output !!
https://www.php.net/manual/en/features.commandline.differences.php

Still show the error: headers have already been sent in ..., when the default behavior is to send the output to terminal, without the need for extra tricks. Using fwrite(), error_handler() or an output buffer.

The php-src code in the CLI SAPI:
https://github.com/php/php-src/blob/master/sapi/cli/php_cli.c#L390-L393

BC: Nothing
In fact we can simplify cli apps by removing the tricks to by pass this problem.

PHP Version

All versions

Operating System

All