Description:
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The ResourceBundle::get() method and the resourcebundle_get() function have an undocumented optional parameter: fallback (bool, default true)

See the doc: http://php.net/manual/en/resourcebundle.get.php

The related code: https://github.com/php/php-src/blob/b78b15e88022b70ce6810f8b8dd1e42c2dac44b3/ext/intl/resourcebundle/resourcebundle_class.c#L259

Test script:
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<?php

$rm = new ReflectionMethod(ResourceBundle::class, 'get');
var_dump($rm->getParameters()[1]->getName());
var_dump($rm->getParameters()[1]->isOptional());

$rf = new ReflectionFunction('resourcebundle_get');
var_dump($rf->getParameters()[2]->getName());
var_dump($rf->getParameters()[2]->isOptional());

Actual result:
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string(8) "fallback"
bool(true)
string(8) "fallback"
bool(true)