Description:
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mb_detect_encoding() seems to have changed quite a bit between PHP 8.0 and 8.1 and mostly for the better, TBH.

However, here's a test case where it misdetects a string as being ASCII when it absolutely cannot be because byte 0 is outside of the 7bit range of ASCII. 

If byte 1 is any non-letter character aside of space, the misdetection will happen. If it's any letter or space, it's fine.

While in <= PHP 8.0, the function had it's quirks, it would never detect a string with a character with its high-bit set as ASCII.

Test script:
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<?php

echo(mb_detect_encoding("\xe4,a", ['ASCII', 'UTF-8', 'ISO-8859-1'])."\n");
echo(mb_detect_encoding("\xe4 a", ['ASCII', 'UTF-8', 'ISO-8859-1'])."\n");


Expected result:
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ISO-8859-1
ISO-8859-1

Actual result:
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ASCII
ISO-8859-1