WordPress/wp-includes/widgets/class-wp-widget-categories.php

<?php
/**
 * Categories widget class
 *
 * @since 2.8.0
 * @package WordPress
 * @subpackage Widgets
 */
class WP_Widget_Categories extends WP_Widget {

	public function __construct() {
		$widget_ops = array( 'classname' => 'widget_categories', 'description' => __( "A list or dropdown of categories." ) );
		parent::__construct('categories', __('Categories'), $widget_ops);
	}

	/**
	 * @staticvar bool $first_dropdown
	 *
	 * @param array $args
	 * @param array $instance
	 */
	public function widget( $args, $instance ) {
		static $first_dropdown = true;

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

		$c = ! empty( $instance['count'] ) ? '1' : '0';
		$h = ! empty( $instance['hierarchical'] ) ? '1' : '0';
		$d = ! empty( $instance['dropdown'] ) ? '1' : '0';

		echo $args['before_widget'];
		if ( $title ) {
			echo $args['before_title'] . $title . $args['after_title'];
		}

		$cat_args = array(
			'orderby'      => 'name',
			'show_count'   => $c,
			'hierarchical' => $h
		);

		if ( $d ) {
			$dropdown_id = ( $first_dropdown ) ? 'cat' : "{$this->id_base}-dropdown-{$this->number}";
			$first_dropdown = false;

			echo '<label class="screen-reader-text" for="' . esc_attr( $dropdown_id ) . '">' . $title . '</label>';

			$cat_args['show_option_none'] = __( 'Select Category' );
			$cat_args['id'] = $dropdown_id;

			/**
			 * Filter the arguments for the Categories widget drop-down.
			 *
			 * @since 2.8.0
			 *
			 * @see wp_dropdown_categories()
			 *
			 * @param array $cat_args An array of Categories widget drop-down arguments.
			 */
			wp_dropdown_categories( apply_filters( 'widget_categories_dropdown_args', $cat_args ) );
?>

<script type='text/javascript'>
/* <![CDATA[ */
(function() {
	var dropdown = document.getElementById( "<?php echo esc_js( $dropdown_id ); ?>" );
	function onCatChange() {
		if ( dropdown.options[ dropdown.selectedIndex ].value > 0 ) {
			location.href = "<?php echo home_url(); ?>/?cat=" + dropdown.options[ dropdown.selectedIndex ].value;
		}
	}
	dropdown.onchange = onCatChange;
})();
/* ]]> */
</script>

<?php
		} else {
?>
		<ul>
<?php
		$cat_args['title_li'] = '';

		/**
		 * Filter the arguments for the Categories widget.
		 *
		 * @since 2.8.0
		 *
		 * @param array $cat_args An array of Categories widget options.
		 */
		wp_list_categories( apply_filters( 'widget_categories_args', $cat_args ) );
?>
		</ul>
<?php
		}

		echo $args['after_widget'];
	}

	/**
	 * @param array $new_instance
	 * @param array $old_instance
	 * @return array
	 */
	public function update( $new_instance, $old_instance ) {
		$instance = $old_instance;
		$instance['title'] = sanitize_text_field( $new_instance['title'] );
		$instance['count'] = !empty($new_instance['count']) ? 1 : 0;
		$instance['hierarchical'] = !empty($new_instance['hierarchical']) ? 1 : 0;
		$instance['dropdown'] = !empty($new_instance['dropdown']) ? 1 : 0;

		return $instance;
	}

	/**
	 * @param array $instance
	 */
	public function form( $instance ) {
		//Defaults
		$instance = wp_parse_args( (array) $instance, array( 'title' => '') );
		$title = sanitize_text_field( $instance['title'] );
		$count = isset($instance['count']) ? (bool) $instance['count'] :false;
		$hierarchical = isset( $instance['hierarchical'] ) ? (bool) $instance['hierarchical'] : false;
		$dropdown = isset( $instance['dropdown'] ) ? (bool) $instance['dropdown'] : false;
?>
		<p><label for="<?php echo $this->get_field_id('title'); ?>"><?php _e( '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( $title ); ?>" /></p>

		<p><input type="checkbox" class="checkbox" id="<?php echo $this->get_field_id('dropdown'); ?>" name="<?php echo $this->get_field_name('dropdown'); ?>"<?php checked( $dropdown ); ?> />
		<label for="<?php echo $this->get_field_id('dropdown'); ?>"><?php _e( 'Display as dropdown' ); ?></label><br />

		<input type="checkbox" class="checkbox" id="<?php echo $this->get_field_id('count'); ?>" name="<?php echo $this->get_field_name('count'); ?>"<?php checked( $count ); ?> />
		<label for="<?php echo $this->get_field_id('count'); ?>"><?php _e( 'Show post counts' ); ?></label><br />

		<input type="checkbox" class="checkbox" id="<?php echo $this->get_field_id('hierarchical'); ?>" name="<?php echo $this->get_field_name('hierarchical'); ?>"<?php checked( $hierarchical ); ?> />
		<label for="<?php echo $this->get_field_id('hierarchical'); ?>"><?php _e( 'Show hierarchy' ); ?></label></p>
<?php
	}

}