WP_Customize_Panel::__construct( WP_Customize_Manager $manager, string $id, array $args = array() )

	public function __construct( $manager, $id, $args = array() ) {
		$keys = array_keys( get_object_vars( $this ) );
		foreach ( $keys as $key ) {
			if ( isset( $args[ $key ] ) ) {
				$this->$key = $args[ $key ];
			}
		}

		$this->manager = $manager;
		$this->id = $id;
		if ( empty( $this->active_callback ) ) {
			$this->active_callback = array( $this, 'active_callback' );
		}
		self::$instance_count += 1;
		$this->instance_number = self::$instance_count;

		$this->sections = array(); // Users cannot customize the $sections array.
	}