Description:
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If either en_US or en are included in the $langtag parameter array, and the locale is empty(), then the en_US or en is returned - regardless of the set default. If neither are in $langtag, then the function acts as expected.

This is similar to Bug #60166, but is different in that having a specific locale in the $langtag changes the behavior from unexpected to buggy.

Test script:
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<?php

$availableLocales = array('fr_FR', 'de', 'es_ES', 'es_419', 'en_US');
echo locale_lookup($availableLocales, false, true, 'fr_FR');
echo locale_lookup($availableLocales, false, true, null);


$availableLocales = array('fr_FR', 'de', 'es_ES', 'es_419');
echo locale_lookup($availableLocales, false, true, 'fr_FR');


Expected result:
----------------
fr_FR
null
fr_FR

Actual result:
--------------
en_us
en_us
fr_FR