Description:
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The IntlChar::charName() method returns bool(false) when an invalid second parameter is given.  According to the method signature, only string values should be returned.

HHVM returns null in these cases, and I think PHP should too.

Also see related issue #70451.

Test script:
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See https://3v4l.org/YXeEg or run the following:

<?php
// Rely on the default value for the second parameter
var_dump(IntlChar::charName("A"));
// Provide a valid option for the second parameter
var_dump(IntlChar::charName("A", IntlChar::UNICODE_CHAR_NAME));
// Another valid option, but with no corresponding name for that given option
// This properly returns an empty string, as expected
var_dump(IntlChar::charName("A", IntlChar::UNICODE_10_CHAR_NAME));
// Provide an invalid value for the second parameter
var_dump(IntlChar::charName("A", 12345));

Expected result:
----------------
string(22) "LATIN CAPITAL LETTER A"
string(22) "LATIN CAPITAL LETTER A"
string(0) ""
NULL

Actual result:
--------------
string(22) "LATIN CAPITAL LETTER A"
string(22) "LATIN CAPITAL LETTER A"
string(0) ""
bool(false)