Those folders contain vanilla PHP code, with NO Debian patches applied.

This is controlled by the display_errors and error_log configuration variables: what do you get on each version if you run php -i | egrep '(display_errors|error_log)' ?

For example, I get:

adamh@swiftdesk7:/tmp$ php -i | egrep '(display_errors|error_log)'
display_errors => STDOUT => STDOUT
error_log => no value => no value

And as you'd expect, errors are output to stdout.

Even though display_errors is set to STDOUT by default in php 5.3.25:

weirdan@367-dsividov:~/Downloads$ php-5.3.25/sapi/cli/php -i -n | grep 
display_errors
display_errors => STDOUT => STDOUT

(and display_startup_errors are off): 

weirdan@367-dsividov:~/Downloads$ php-5.3.25/sapi/cli/php -i -n | grep 
display_startup
display_startup_errors => Off => Off
weirdan@367-dsividov:~/Downloads$

the following 

weirdan@367-dsividov:~/Downloads$ php-5.3.25/sapi/cli/php -n $(tempfile).php 
2>/dev/null
weirdan@367-dsividov:~/Downloads$ php-5.3.25/sapi/cli/php -n $(tempfile).php
Could not open input file: /tmp/fileLHIKSE.php
weirdan@367-dsividov:~/Downloads$

clearly shows that startup errors are actually going to STDERR

But even I specify errors should go to STDERR, they go to STDOUT in php >= 
5.4.0:


weirdan@367-dsividov:~/Downloads$ for dir in `ls -d php-5* | grep -v tar`; do 
echo -n "$dir: "; $dir/sapi/cli/php -ddisplay_errors=STDERR -
ddisplay_startup_errors=0 -n $(tempfile).php 2>/dev/null; echo; done;

php-5.3.25:
php-5.4.0: Could not open input file: /tmp/filedhceXV.php

php-5.4.15: Could not open input file: /tmp/fileUv3FxW.php

php-5.4.7: Could not open input file: /tmp/fileBH6TsZ.php

weirdan@367-dsividov:~/Downloads$



to me it seems that startup errors started to go to STDOUT (regardless of 
display_errors and display_startup_errors value) in php-5.4.x and you can't 
really control it through either 
display_errors or display_startup_errors:

weirdan@367-dsividov:~/Downloads$ php-5.4.0/sapi/cli/php -
ddisplay_startup_errors=STDERR -ddisplay_errors=STDERR -i | egrep 
'(display_errors|display_startup_errors|error_log)'
display_errors => STDERR => STDERR
display_startup_errors => Off => Off
error_log => no value => no value
weirdan@367-dsividov:~/Downloads$ php-5.3.25/sapi/cli/php -
ddisplay_startup_errors=STDERR -ddisplay_errors=STDERR -i | egrep 
'(display_errors|display_startup_errors|error_log)'
display_errors => STDERR => STDERR
display_startup_errors => Off => Off
error_log => no value => no value
weirdan@367-dsividov:~/Downloads$ php-5.4.0/sapi/cli/php -n -
ddisplay_startup_errors=STDERR -ddisplay_errors=STDERR $(tempfile).php 
2>/dev/null
Could not open input file: /tmp/file7laZhC.php
weirdan@367-dsividov:~/Downloads$ php-5.3.25/sapi/cli/php -n -
ddisplay_startup_errors=STDERR -ddisplay_errors=STDERR $(tempfile).php 
2>/dev/null
weirdan@367-dsividov:~/Downloads$

I can confirm this issue on 5.6.0RC1:

$ ./inst/bin/php-5.6.0RC1 -i|grep display_errors
display_errors => STDERR => STDERR
$ ./inst/bin/php-5.6.0RC1 foo.php > /dev/null
$

I can confirm this issue still exists with latest master. The
culprit appears to be that php_printf() is used[1].

[1] <https://github.com/php/php-src/blob/php-7.0.7/sapi/cli/php_cli.c#L622>

I can confirm that the issue still persists in version 7.4.9.

[user.name@server ~]$ php -v
PHP 7.4.9 (cli) (built: Aug  4 2020 08:28:13) ( NTS )
...

[user.name@server ~]$ php -i | egrep '(display_errors|error_log)'
display_errors => STDERR => STDERR
error_log => no value => no value
opcache.error_log => no value => no value

[user.name@server ~]$ php non-existing-file.php > /dev/null

[user.name@server ~]$ php non-existing-file.php
Could not open input file: non-existing-file.php