Source

File: wp-includes/post-template.php

function wp_dropdown_pages( $args = '' ) {
	$defaults = array(
		'depth'                 => 0,
		'child_of'              => 0,
		'selected'              => 0,
		'echo'                  => 1,
		'name'                  => 'page_id',
		'id'                    => '',
		'class'                 => '',
		'show_option_none'      => '',
		'show_option_no_change' => '',
		'option_none_value'     => '',
		'value_field'           => 'ID',
	);

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

	$pages  = get_pages( $parsed_args );
	$output = '';
	// Back-compat with old system where both id and name were based on $name argument.
	if ( empty( $parsed_args['id'] ) ) {
		$parsed_args['id'] = $parsed_args['name'];
	}

	if ( ! empty( $pages ) ) {
		$class = '';
		if ( ! empty( $parsed_args['class'] ) ) {
			$class = " class='" . esc_attr( $parsed_args['class'] ) . "'";
		}

		$output = "<select name='" . esc_attr( $parsed_args['name'] ) . "'" . $class . " id='" . esc_attr( $parsed_args['id'] ) . "'>\n";
		if ( $parsed_args['show_option_no_change'] ) {
			$output .= "\t<option value=\"-1\">" . $parsed_args['show_option_no_change'] . "</option>\n";
		}
		if ( $parsed_args['show_option_none'] ) {
			$output .= "\t<option value=\"" . esc_attr( $parsed_args['option_none_value'] ) . '">' . $parsed_args['show_option_none'] . "</option>\n";
		}
		$output .= walk_page_dropdown_tree( $pages, $parsed_args['depth'], $parsed_args );
		$output .= "</select>\n";
	}

	/**
	 * Filters the HTML output of a list of pages as a drop down.
	 *
	 * @since 2.1.0
	 * @since 4.4.0 `$parsed_args` and `$pages` added as arguments.
	 *
	 * @param string    $output      HTML output for drop down list of pages.
	 * @param array     $parsed_args The parsed arguments array.
	 * @param WP_Post[] $pages       Array of the page objects.
	 */
	$html = apply_filters( 'wp_dropdown_pages', $output, $parsed_args, $pages );

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

	return $html;
}