Source

File: wp-includes/user.php

function wp_dropdown_users( $args = '' ) {
	$defaults = array(
		'show_option_all'         => '',
		'show_option_none'        => '',
		'hide_if_only_one_author' => '',
		'orderby'                 => 'display_name',
		'order'                   => 'ASC',
		'include'                 => '',
		'exclude'                 => '',
		'multi'                   => 0,
		'show'                    => 'display_name',
		'echo'                    => 1,
		'selected'                => 0,
		'name'                    => 'user',
		'class'                   => '',
		'id'                      => '',
		'blog_id'                 => get_current_blog_id(),
		'who'                     => '',
		'include_selected'        => false,
		'option_none_value'       => -1,
		'role'                    => '',
		'role__in'                => array(),
		'role__not_in'            => array(),
	);

	$defaults['selected'] = is_author() ? get_query_var( 'author' ) : 0;

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

	$query_args = wp_array_slice_assoc( $parsed_args, array( 'blog_id', 'include', 'exclude', 'orderby', 'order', 'who', 'role', 'role__in', 'role__not_in' ) );

	$fields = array( 'ID', 'user_login' );

	$show = ! empty( $parsed_args['show'] ) ? $parsed_args['show'] : 'display_name';
	if ( 'display_name_with_login' === $show ) {
		$fields[] = 'display_name';
	} else {
		$fields[] = $show;
	}

	$query_args['fields'] = $fields;

	$show_option_all   = $parsed_args['show_option_all'];
	$show_option_none  = $parsed_args['show_option_none'];
	$option_none_value = $parsed_args['option_none_value'];

	/**
	 * Filters the query arguments for the list of users in the dropdown.
	 *
	 * @since 4.4.0
	 *
	 * @param array $query_args  The query arguments for get_users().
	 * @param array $parsed_args The arguments passed to wp_dropdown_users() combined with the defaults.
	 */
	$query_args = apply_filters( 'wp_dropdown_users_args', $query_args, $parsed_args );

	$users = get_users( $query_args );

	$output = '';
	if ( ! empty( $users ) && ( empty( $parsed_args['hide_if_only_one_author'] ) || count( $users ) > 1 ) ) {
		$name = esc_attr( $parsed_args['name'] );
		if ( $parsed_args['multi'] && ! $parsed_args['id'] ) {
			$id = '';
		} else {
			$id = $parsed_args['id'] ? " id='" . esc_attr( $parsed_args['id'] ) . "'" : " id='$name'";
		}
		$output = "<select name='{$name}'{$id} class='" . $parsed_args['class'] . "'>\n";

		if ( $show_option_all ) {
			$output .= "\t<option value='0'>$show_option_all</option>\n";
		}

		if ( $show_option_none ) {
			$_selected = selected( $option_none_value, $parsed_args['selected'], false );
			$output   .= "\t<option value='" . esc_attr( $option_none_value ) . "'$_selected>$show_option_none</option>\n";
		}

		if ( $parsed_args['include_selected'] && ( $parsed_args['selected'] > 0 ) ) {
			$found_selected          = false;
			$parsed_args['selected'] = (int) $parsed_args['selected'];
			foreach ( (array) $users as $user ) {
				$user->ID = (int) $user->ID;
				if ( $user->ID === $parsed_args['selected'] ) {
					$found_selected = true;
				}
			}

			if ( ! $found_selected ) {
				$users[] = get_userdata( $parsed_args['selected'] );
			}
		}

		foreach ( (array) $users as $user ) {
			if ( 'display_name_with_login' === $show ) {
				/* translators: 1: User's display name, 2: User login. */
				$display = sprintf( _x( '%1$s (%2$s)', 'user dropdown' ), $user->display_name, $user->user_login );
			} elseif ( ! empty( $user->$show ) ) {
				$display = $user->$show;
			} else {
				$display = '(' . $user->user_login . ')';
			}

			$_selected = selected( $user->ID, $parsed_args['selected'], false );
			$output   .= "\t<option value='$user->ID'$_selected>" . esc_html( $display ) . "</option>\n";
		}

		$output .= '</select>';
	}

	/**
	 * Filters the wp_dropdown_users() HTML output.
	 *
	 * @since 2.3.0
	 *
	 * @param string $output HTML output generated by wp_dropdown_users().
	 */
	$html = apply_filters( 'wp_dropdown_users', $output );

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