Source

File: wp-includes/class-wp-ajax-response.php

public function add( $args = '' ) {
		$defaults = array(
			'what'         => 'object',
			'action'       => false,
			'id'           => '0',
			'old_id'       => false,
			'position'     => 1,
			'data'         => '',
			'supplemental' => array(),
		);

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

		$position = preg_replace( '/[^a-z0-9:_-]/i', '', $parsed_args['position'] );
		$id       = $parsed_args['id'];
		$what     = $parsed_args['what'];
		$action   = $parsed_args['action'];
		$old_id   = $parsed_args['old_id'];
		$data     = $parsed_args['data'];

		if ( is_wp_error( $id ) ) {
			$data = $id;
			$id   = 0;
		}

		$response = '';
		if ( is_wp_error( $data ) ) {
			foreach ( (array) $data->get_error_codes() as $code ) {
				$response  .= "<wp_error code='$code'><![CDATA[" . $data->get_error_message( $code ) . ']]></wp_error>';
				$error_data = $data->get_error_data( $code );
				if ( ! $error_data ) {
					continue;
				}
				$class = '';
				if ( is_object( $error_data ) ) {
					$class      = ' class="' . get_class( $error_data ) . '"';
					$error_data = get_object_vars( $error_data );
				}

				$response .= "<wp_error_data code='$code'$class>";

				if ( is_scalar( $error_data ) ) {
					$response .= "<![CDATA[$error_data]]>";
				} elseif ( is_array( $error_data ) ) {
					foreach ( $error_data as $k => $v ) {
						$response .= "<$k><![CDATA[$v]]></$k>";
					}
				}

				$response .= '</wp_error_data>';
			}
		} else {
			$response = "<response_data><![CDATA[$data]]></response_data>";
		}

		$s = '';
		if ( is_array( $parsed_args['supplemental'] ) ) {
			foreach ( $parsed_args['supplemental'] as $k => $v ) {
				$s .= "<$k><![CDATA[$v]]></$k>";
			}
			$s = "<supplemental>$s</supplemental>";
		}

		if ( false === $action ) {
			$action = $_POST['action'];
		}
		$x  = '';
		$x .= "<response action='{$action}_$id'>"; // The action attribute in the xml output is formatted like a nonce action.
		$x .= "<$what id='$id' " . ( false === $old_id ? '' : "old_id='$old_id' " ) . "position='$position'>";
		$x .= $response;
		$x .= $s;
		$x .= "</$what>";
		$x .= '</response>';

		$this->responses[] = $x;
		return $x;
	}