Description:
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The function FILTER_SANITIZE_FULL_SPECIAL_CHARS doesn't returns the chars equivalent to calling htmlspecialchars() with ENT_QUOTES set. 

php.ini:
default_charset = "utf-8" 

Test script:
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<?php
$string = '?';

echo 'filter_var: ' . filter_var($string, FILTER_SANITIZE_FULL_SPECIAL_CHARS) . PHP_EOL;
echo 'htmlspecialchars: ' . htmlspecialchars($string, ENT_QUOTES);

Expected result:
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filter_var: ?
htmlspecialchars: ?

Actual result:
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filter_var: �
htmlspecialchars: ?