system/Router/RouteCollection.php
public function setDefaultController(string $value): RouteCollectionInterface
	{
		$this->defaultController = filter_var($value, FILTER_SANITIZE_STRING);

		return $this;
	}
public function setDefaultController(string $value): RouteCollectionInterface
	{
		$this->defaultController = filter_var($value, FILTER_SANITIZE_STRING);

		return $this;
	}
Sets the default controller to use when no other controller has been
specified.

@param $value

@return \CodeIgniter\Router\RouteCollectionInterface