How to get the type hints of native PHP class constructors

When using the ReflectionClass to loop through native PHP class __construct() parameters, I've noticed that the names of the parameters in the docs aren't matching up to what I'm getting using the ReflectionParameter class methods.

For example the DateTime object:

$reflector = new ReflectionClass('DateTime');
$constructor = $reflector->getConstructor();

if ($constructorParams = $constructor->getParameters()) {
    foreach ($constructorParams as $i => $param) {
        var_dump($param->getClass());
    }
}

This outputs:

null
null

But according to the docs the class description is:

public DateTime::__construct ([ string $time = "now" [, DateTimeZone $timezone = NULL ]] )

Shouldn't the $timezone parameter have a type hint of DateTimeZone? Or is the parameter not actually type hinted, and rather just telling you what should be passed in?