Description:
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When the original sring $str is longer then $width of the desired trim, the width of the truncated string returned by mb_strimwidth() will count "length of $trimmarker" in.

Reproduce code:
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echo mb_strimwidth('helloworld', 0, 5, '...', 'UTF-8');
echo mb_strimwidth('hello', 0, 5, '...', 'UTF-8');

Expected result:
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hello...
hello

Actual result:
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he...
hello