XMLWriter::startDocument() writes the XML declaration.Without XMLWriter::startDocument() :<?php$xml = new XMLWriter();$xml->openUri('php://stdout');$xml->writeElement('message', 'Hello World!');exit;/*Outputs :<message>Hello World!</message>*/?>With XMLWriter::startDocument() :<?php$xml = new XMLWriter();$xml->openUri('php://stdout');$xml->startDocument();$xml->writeElement('message', 'Hello World!');exit;/*Outputs :<?xml version="1.0"?><message>Hello World!</message>*/?>