Description:
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I have found that mb_check_encoding returns false when I pass it a string that I think is correctly encoded. It occurs when the string contains Roman characters encoded in the JIS X 0201-1976 character set. I cehcked that according to RFC 1468 this encoding is valid. Furthermore, mb_convert_encoding is able to decode this string.

Reproduce code:
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$encoded = "\x1b(JTEST";
if(!mb_check_encoding($encoded,'ISO-2022-JP')) echo "check encoding failed\n";
echo mb_convert_encoding($encoded,'UTF-8','ISO-2022-JP')."\n";

Expected result:
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I expect only the word "TEST" to be printed

Actual result:
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"check encoding failed" is printed indicating that mb_check_encoding returned FALSE. "TEST" is also printed, showing that mb_chonvert_encoding successfully decoded the string.