Description:
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all string related function i did test (substr, mb_substr, iconv_substr) return a empty string when the result should be one space character.

Test script:
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// Source code saved in UTF-8 encoding.
$string = 'a b';
$c1 = substr($string, 1, 1);
$c2 = mb_substr($string, 1, 1);
$c3 = iconv_substr($string, 1, 1);
// then we obtain:
// $c1 == $c2 == $c3 == ""
// instead of 
// $c1 == $c2 == $c3 == " "