Various output buffer issues

[kkmuffme]

Q	A
PHPUnit version	11.1.3
PHP version	8.2.x, 8.3.x
Installation Method	Composer / PHAR

Summary

1. when a test (or any function called from the test) flushes or cleans the output buffer, this incorrectly changes the output making tests pass that fail with expectOutputString and/or would cause a PHP notice when the code is actually executed
2. if a test ends but starts a new output buffer, no error is reported, since only the ob level is compared instead of comparing the output
3. when a test (or any function called from the test) register a non-removable ob callback, this leads to complete failure/unexpected behavior
4. non-removable ob callbacks cannot currently be tested
5. if stopping output buffering fails, it will try again creating the same notices twice

I will provide a PR.

[sebastianbergmann]

What are non-removable output buffering handlers?

[kkmuffme]

e.g.

• ob_start('my_cb', 0, 0);
• ob_start('my_cb', 0, PHP_OUTPUT_HANDLER_FLUSHABLE);
• ob_start(null, 0, PHP_OUTPUT_HANDLER_CLEANABLE);

These cannot be removed/terminated with e.g. ob_end_flush();

[mvorisek]

Can you show a small repro on https://3v4l.org/?

[kkmuffme]

https://3v4l.org/kPQXO

See the PR, there are unit tests for that case already
https://github.com/sebastianbergmann/phpunit/pull/5852/files#diff-198b09778388878e8068a870068387c10c3f9db2a6ed610c24daccda3b669fa3R115