Description:
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If I try to use almost any string function on a string with non-ascii characters encoded in UTF-8 I get some messed up results.

This relates mainly to: substr, str_replace, strtr, strtolower, strtoupper, strlen, ...

My string: consists of czech, slovak, german, etc. chars like ? (č) , ň (ň) ? (ä) , ? (ö) ,...

I can't even use the most basic thing like below.

Reproduce code:
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$string = "aa?"; // last character is special czech "c" (č) 
echo $string[2];

Expected result:
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// the code should just print "?" (č)
?

Actual result:
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// it prints some character (depends on the character following "?"
?