Description of ENT_DISALLOWED in htmlentities is misleading

[mondolirondo]

From manual page: https://php.net/function.htmlentities

Description of ENT_DISALLOWED is misleading.
It states: "Replace invalid code points for the given document type with a Unicode Replacement Character U+FFFD (UTF-8) or &#FFFD; (otherwise) instead of leaving them as is. This may be useful, for instance, to ensure the well-formedness of XML documents with embedded external content."
It should say somenthing like: "Returns an empty string when invalid code points for the given document are found."

Tested using the following code:

$string = "This is a test: \x80\x81";
echo htmlentities($string, ENT_SUBSTITUTE, 'UTF-8');
// Output: This is a test: ��
echo "\n";
$string = "This is a test: \x80\x81";
echo htmlentities($string, ENT_DISALLOWED, 'UTF-8');
// Output: <empty line> 
echo "\n";
$string = "This is a test: xx";
echo htmlentities($string, ENT_DISALLOWED, 'UTF-8');
// Output: This is a test: xx