Source

File: wp-admin/includes/template.php

function wp_terms_checklist( $post_id = 0, $args = array() ) {
	$defaults = array(
		'descendants_and_self' => 0,
		'selected_cats'        => false,
		'popular_cats'         => false,
		'walker'               => null,
		'taxonomy'             => 'category',
		'checked_ontop'        => true,
		'echo'                 => true,
	);

	/**
	 * Filters the taxonomy terms checklist arguments.
	 *
	 * @since 3.4.0
	 *
	 * @see wp_terms_checklist()
	 *
	 * @param array $args    An array of arguments.
	 * @param int   $post_id The post ID.
	 */
	$params = apply_filters( 'wp_terms_checklist_args', $args, $post_id );

	$parsed_args = wp_parse_args( $params, $defaults );

	if ( empty( $parsed_args['walker'] ) || ! ( $parsed_args['walker'] instanceof Walker ) ) {
		$walker = new Walker_Category_Checklist;
	} else {
		$walker = $parsed_args['walker'];
	}

	$taxonomy             = $parsed_args['taxonomy'];
	$descendants_and_self = (int) $parsed_args['descendants_and_self'];

	$args = array( 'taxonomy' => $taxonomy );

	$tax              = get_taxonomy( $taxonomy );
	$args['disabled'] = ! current_user_can( $tax->cap->assign_terms );

	$args['list_only'] = ! empty( $parsed_args['list_only'] );

	if ( is_array( $parsed_args['selected_cats'] ) ) {
		$args['selected_cats'] = array_map( 'intval', $parsed_args['selected_cats'] );
	} elseif ( $post_id ) {
		$args['selected_cats'] = wp_get_object_terms( $post_id, $taxonomy, array_merge( $args, array( 'fields' => 'ids' ) ) );
	} else {
		$args['selected_cats'] = array();
	}

	if ( is_array( $parsed_args['popular_cats'] ) ) {
		$args['popular_cats'] = array_map( 'intval', $parsed_args['popular_cats'] );
	} else {
		$args['popular_cats'] = get_terms(
			array(
				'taxonomy'     => $taxonomy,
				'fields'       => 'ids',
				'orderby'      => 'count',
				'order'        => 'DESC',
				'number'       => 10,
				'hierarchical' => false,
			)
		);
	}

	if ( $descendants_and_self ) {
		$categories = (array) get_terms(
			array(
				'taxonomy'     => $taxonomy,
				'child_of'     => $descendants_and_self,
				'hierarchical' => 0,
				'hide_empty'   => 0,
			)
		);
		$self       = get_term( $descendants_and_self, $taxonomy );
		array_unshift( $categories, $self );
	} else {
		$categories = (array) get_terms(
			array(
				'taxonomy' => $taxonomy,
				'get'      => 'all',
			)
		);
	}

	$output = '';

	if ( $parsed_args['checked_ontop'] ) {
		// Post-process $categories rather than adding an exclude to the get_terms() query
		// to keep the query the same across all posts (for any query cache).
		$checked_categories = array();
		$keys               = array_keys( $categories );

		foreach ( $keys as $k ) {
			if ( in_array( $categories[ $k ]->term_id, $args['selected_cats'], true ) ) {
				$checked_categories[] = $categories[ $k ];
				unset( $categories[ $k ] );
			}
		}

		// Put checked categories on top.
		$output .= $walker->walk( $checked_categories, 0, $args );
	}
	// Then the rest of them.
	$output .= $walker->walk( $categories, 0, $args );

	if ( $parsed_args['echo'] ) {
		echo $output;
	}

	return $output;
}