Source

class WP_Widget_Calendar extends WP_Widget {
	/**
	 * Ensure that the ID attribute only appears in the markup once
	 *
	 * @since 4.4.0
	 * @var int
	 */
	private static $instance = 0;

	/**
	 * Sets up a new Calendar widget instance.
	 *
	 * @since 2.8.0
	 */
	public function __construct() {
		$widget_ops = array(
			'classname'                   => 'widget_calendar',
			'description'                 => __( 'A calendar of your site’s posts.' ),
			'customize_selective_refresh' => true,
			'show_instance_in_rest'       => true,
		);
		parent::__construct( 'calendar', __( 'Calendar' ), $widget_ops );
	}

	/**
	 * Outputs the content for the current Calendar widget instance.
	 *
	 * @since 2.8.0
	 *
	 * @param array $args     Display arguments including 'before_title', 'after_title',
	 *                        'before_widget', and 'after_widget'.
	 * @param array $instance The settings for the particular instance of the widget.
	 */
	public function widget( $args, $instance ) {
		$title = ! empty( $instance['title'] ) ? $instance['title'] : '';

		/** This filter is documented in wp-includes/widgets/class-wp-widget-pages.php */
		$title = apply_filters( 'widget_title', $title, $instance, $this->id_base );

		echo $args['before_widget'];
		if ( $title ) {
			echo $args['before_title'] . $title . $args['after_title'];
		}
		if ( 0 === self::$instance ) {
			echo '<div id="calendar_wrap" class="calendar_wrap">';
		} else {
			echo '<div class="calendar_wrap">';
		}
		get_calendar();
		echo '</div>';
		echo $args['after_widget'];

		++self::$instance;
	}

	/**
	 * Handles updating settings for the current Calendar widget instance.
	 *
	 * @since 2.8.0
	 *
	 * @param array $new_instance New settings for this instance as input by the user via
	 *                            WP_Widget::form().
	 * @param array $old_instance Old settings for this instance.
	 * @return array Updated settings to save.
	 */
	public function update( $new_instance, $old_instance ) {
		$instance          = $old_instance;
		$instance['title'] = sanitize_text_field( $new_instance['title'] );

		return $instance;
	}

	/**
	 * Outputs the settings form for the Calendar widget.
	 *
	 * @since 2.8.0
	 *
	 * @param array $instance Current settings.
	 */
	public function form( $instance ) {
		$instance = wp_parse_args( (array) $instance, array( 'title' => '' ) );
		?>
		<p>
			<label for="<?php echo $this->get_field_id( 'title' ); ?>"></label>
			<input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $instance['title'] ); ?>" />
		</p>
		</pre><p class="wporg-dot-link-list"><a href="https://developer.wordpress.org/reference/files/wp-includes/widgets/class-wp-widget-calendar.php/">View all references</a> <a href="https://core.trac.wordpress.org/browser/tags/6.9.4/src/wp-includes/widgets/class-wp-widget-calendar.php#L17">View on Trac</a> <a href="https://github.com/WordPress/wordpress-develop/blob/6.9.4/src/wp-includes/widgets/class-wp-widget-calendar.php#L17-L105">View on GitHub</a></p></section>
		
		<section class="wp-block-wporg-code-reference-related" data-nosnippet="true"><h2 id="related" class="is-toc-heading wp-block-heading has-heading-5-font-size" tabindex="-1" ><a href="#related">Related</a></h2> <section style="margin-top:var(--wp--preset--spacing--20)" class="wp-block-wporg-code-table" id="uses"><figure class="wp-block-table "><table><thead><tr><th scope="col">Uses</th><th scope="col">Description</th></tr></thead><tbody><tr class=""><td><a href="https://developer.wordpress.org/reference/classes/wp_widget/">WP_Widget</a><code>wp-includes/class-wp-widget.php
Core base class extended to register widgets.