How does the lack of a <!DOCTYPE...> directive impact the validation of XML documents in PHP and web browsers?

Without a <!DOCTYPE...> directive, XML documents may not be validated correctly in PHP and web browsers. The <!DOCTYPE...> declaration specifies the document type definition (DTD) that defines the structure and rules for the XML document. Without it, the validation process may not be able to determine if the document follows the correct structure and rules. To ensure proper validation of XML documents in PHP and web browsers, it is important to include a <!DOCTYPE...> declaration at the beginning of the XML document. This declaration should specify the appropriate DTD for the document.

<?php
// Sample XML document with <!DOCTYPE...> declaration
$xml = '<?xml version="1.0"?>
<!DOCTYPE example SYSTEM "example.dtd">
<example>
  <item>Item 1</item>
  <item>Item 2</item>
</example>';

// Load the XML document
$doc = new DOMDocument();
$doc->loadXML($xml);

// Validate the XML document
if ($doc->validate()) {
  echo 'XML document is valid.';
} else {
  echo 'XML document is not valid.';
}
?>