Description:
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From manual page: http://www.php.net/function.htmlspecialchars#Parameters
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For the purposes of this function, the charsets ISO-8859-1, ISO-8859-15, UTF-8, cp866, cp1251, cp1252, and KOI8-R are effectively equivalent, as the characters affected by htmlspecialchars() occupy the same positions in all of these charsets.

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This is misleading. (The paragraph was introduced by #49412.)

Example:

var_dump( htmlspecialchars( "\xe8", ENT_QUOTES, 'UTF-8' ) ) returns:

PHP 5.2.1:  string(1) "�"

but in later versions:

PHP 5.2.6:  string(0) ""
PHP 5.3.5:  string(0) ""


Test script:
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echo htmlspecialchars( "\xe8", ENT_QUOTES, 'UTF-8' );


Expected result:
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I suggest removing the aforementioned paragraph from the manpage, and add a note under Changelog or Return Values, stating that, as of php 5.2.5 (judging from php.net/ChangeLog-5.php), the function can return an empty string if there are invalid byte sequences for the specified charset.