Description:
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mb_detect_encoding() according to the documentation should return "false if the string is not valid in any of the listed encodings".

However, the test script shows that this is not the case.

https://3v4l.org/km1R7

Related to bug #48210 which has been closed without proper evaluation, imo. I hope, my test script is more clear about the issue.

Test script:
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$test = quoted_printable_decode('Piotr Iksi=F1ski');
echo mb_check_encoding($test, 'UTF-8') ? "valid:" : "invalid:";
echo mb_detect_encoding($test, 'ASCII,UTF-8') ?: 'false';

Expected result:
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invalid:false

Actual result:
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invalid:UTF-8