Description:
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From manual page: http://www.php.net/reflectionparameter.iscallable
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Bug for internal function - callable set_exception_handler ( callable $exception_handler )
not work ReflectionParameter::isCallable

Test script:
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$refl = new ReflectionFunction('set_exception_handler');

$parameters = $refl->getParameters();

var_dump($parameters[0]->getName(), $parameters[0]->isCallable());

Expected result:
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string 'exception_handler' (length=17)
boolean false