[Debug] work-around https://bugs.php.net/61272
bug #11099 [Debug] work-around https://bugs.php.net/61272 (nicolas-grekas)

This PR was merged into the 2.5 branch.

Discussion
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[Debug] work-around https://bugs.php.net/61272

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8703
| License       | MIT

https://bugs.php.net/61272 is the real offender.
Using ob_end_flush() instead of ob_end_clean() is the fix.

Commits
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cb8aff3 [Debug] work-around https://bugs.php.net/61272