Description:
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"mb_strpos()" is not compatible with old PHP versions, because it dons't trigger a warning anymore "Offset not contained in string"?!

-> https://3v4l.org/o1YNG

Test script:
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var_dump(mb_strpos('abc', 'a', -1));

Expected result:
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Warning: mb_strpos(): Offset not contained in string in /in/o1YNG on line 3
bool(false)

Actual result:
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bool(false)