JezK

/**
 * Core class used to create an HTML dropdown list of Categories.
 *
 * @since 2.1.0
 *
 * @see Walker
 */
class Walker_CategoryDropdown extends Walker {

/**
	 * What the class handles.
	 *
	 * @since 2.1.0
	 * @var string
	 *
	 * @see Walker::$tree_type
	 */
	public $tree_type = 'category';

/**
	 * Database fields to use.
	 *
	 * @since 2.1.0
	 * @todo Decouple this
	 * @var string[]
	 *
	 * @see Walker::$db_fields
	 */
	public $db_fields = array(
		'parent' => 'parent',
		'id'     => 'term_id',
	);

/**
	 * Starts the element output.
	 *
	 * @since 2.1.0
	 * @since 5.9.0 Renamed `$category` to `$data_object` and `$id` to `$current_object_id`
	 *              to match parent class for PHP 8 named parameter support.
	 *
	 * @see Walker::start_el()
	 *
	 * @param string  $output            Used to append additional content (passed by reference).
	 * @param WP_Term $data_object       Category data object.
	 * @param int     $depth             Depth of category. Used for padding.
	 * @param array   $args              Uses 'selected', 'show_count', and 'value_field' keys, if they exist.
	 *                                   See wp_dropdown_categories().
	 * @param int     $current_object_id Optional. ID of the current category. Default 0.
	 */
	public function start_el( &$output, $data_object, $depth = 0, $args = array(), $current_object_id = 0 ) {
		// Restores the more descriptive, specific name for use within this method.
		$category = $data_object;

$pad = str_repeat( '&nbsp;', $depth * 3 );

/** This filter is documented in wp-includes/category-template.php */
		$cat_name = apply_filters( 'list_cats', $category->name, $category );

if ( isset( $args['value_field'] ) && isset( $category->{$args['value_field']} ) ) {
			$value_field = $args['value_field'];
		} else {
			$value_field = 'term_id';
		}

$output .= "\t<option class=\"level-$depth\" value=\"" . esc_attr( $category->{$value_field} ) . '"';

// Type-juggling causes false matches, so we force everything to a string.
		if ( (string) $category->{$value_field} === (string) $args['selected'] ) {
			$output .= ' selected="selected"';
		}
		$output .= '>';
		$output .= $pad . $cat_name;
		if ( $args['show_count'] ) {
			$output .= '&nbsp;&nbsp;(' . number_format_i18n( $category->count ) . ')';
		}
		$output .= "</option>\n";
	}
}

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Edit File: class-walker-category-dropdown.php