What is the difference between XML and XHTML in terms of validation requirements and namespace usage?

XML is a markup language that allows for the creation of custom tags and attributes, while XHTML is a stricter version of HTML that follows stricter rules and syntax. In terms of validation requirements, XHTML requires documents to be well-formed and valid according to the rules of XML, while XML does not have the same strict validation requirements. Additionally, XHTML uses the XHTML namespace to define its elements, while XML does not have a specific namespace requirement.

// Example of creating an XHTML document with proper validation requirements and namespace usage
$xml = new DOMDocument('1.0', 'UTF-8');
$xml->validateOnParse = true;

// Create root element with XHTML namespace
$root = $xml->createElementNS('http://www.w3.org/1999/xhtml', 'html');
$xml->appendChild($root);

// Create child elements with proper namespace
$body = $xml->createElementNS('http://www.w3.org/1999/xhtml', 'body');
$root->appendChild($body);

// Output the XML document
echo $xml->saveXML();