Description:
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Reproducible on Amazon Linux, Slackware and others with various combinations of (recent) PHP and Apache. When httpd.conf is configured with an ErrorDocument to a PHP script and an error is deliberately introduced with a faulty header, the mere presence of a flush() changes the graceful error handling (via a subrequest) in to a confusing and bizarre situation that introduces secondary bugs.

These include:
o PHP no longer honours previous require_once include guard
o Apache not sending back any output to the client
o Misleading messages in the PHP error log about other modules
o Misleading messages in the PHP error log about ini_set (?) which is never used
o PHP error log messages repeated multiple times

The presence of a single flush() changes the graceful error handling ($test = 2) in to a confusing mess ($test = 3) and introduces seemingly unrelated bugs.

Steps to reproduce:
1. Edit an Apache VirtualHost config in to include the following:
  ErrorDocument 500 /error.php
2. Place the 3 attached scripts in the VirtualHost's DocumentRoot:
  test.php error.php library.php
3. Restart Apache
4. Access https://somesite/test.php
5. Alter test.php to try $test = 2 and $test = 3
6. Observe all the unexpected and secondary problems the flush() introduces

Test script:
---------------
https://pastebin.com/MBQEEb5Q

Expected result:
----------------
Expected result for test 3 is same as test 2:

----

PHP error log:
No new entries.

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$ curl -i https://somesite/test.php

HTTP/1.1 500 Internal Server Error
Date: Sun, 27 Dec 2020 14:05:47 GMT
Server: Apache
Content-Type: text/html; charset=UTF-8

I will deal with a 500 error and show a pretty error message

Actual result:
--------------
Actual result for test 3:

----

PHP error log:

[27-Dec-2020 13:49:41 UTC] PHP Warning:  flush(): Headers already sent. You cannot change the session module's ini settings at this time in /www/test.php on line 22
[27-Dec-2020 13:49:41 UTC] PHP Warning:  flush(): Headers already sent. You cannot change the session module's ini settings at this time in /www/test.php on line 22
[27-Dec-2020 13:49:41 UTC] PHP Warning:  flush(): Headers already sent. You cannot change the session module's ini settings at this time in /www/test.php on line 22
[27-Dec-2020 13:49:41 UTC] PHP Warning:  flush(): Headers already sent. You cannot change the session module's ini settings at this time in /www/test.php on line 22
[27-Dec-2020 13:49:41 UTC] PHP Fatal error:  Cannot redeclare foo() (previously declared in /www/library.php:4) in /www/library.php on line 4

----

$ curl -i https://somesite/test.php

curl: (52) Empty reply from server