XSD: Validate Extension XML

extension.about.xml

<?xml version="1.0" encoding="UTF-8" ?>

<extension id="your_extension_path" xmlns="http://symphony-cms.com/extensions/1.0">

	<!-- string, required -->
	<name>Your Extension Name</name>

	<!-- string, required (@lang optional) -->
	<description lang="en">Add the description to your extension here.</description>

	<!-- string, required (@type=github|codebase|other, for future repo posibilities?) (must be Github for clever API things to happen) -->
	<repo type="github">https://github.com/username/your_extension_path</repo>

	<!-- string, optional (@type=homepage|wiki|other?) -->
	<url type="homepage">http://symphony-cms.com/downloads/extensions/your_extension_path</url>

	<!-- list, optional (Symphony to define suggested vocabulary, suggest leaving this out initially?) -->
	<types>
		<type>Event</type>
		<type>Field</type>
		<type>Interface</type>
		<type>Multimedia</type>
		<type>Text Formatter</type>
		<type>Translation</type>
		<type>Workflow</type>
		<type>Other</type>
	</types>

	<!-- list, required -->
	<authors>
		<author>
			<!-- string, required (@github, @symphony, @twitter optional) -->
			<name github="username" symphony="username" twitter="username">Your Name</name>
			<!-- string, optional. If used must validate as email -->
			<email>name@domain.co.uk</email>
			<!-- string, optional. If used must validate with http:// -->
			<website>http://domain.co.uk</website>
		</author>
	</authors>

	<!-- list, optional -->
	<dependencies>
		<dependency version="0.5.0">extension_path</dependency>
	</dependencies>

	<!-- list, required at least one. Largest version number is assumed latest -->
	<releases>
		<release version="0.3" date="2011-03-03" min="2.3" max="2.4"/>
		<release version="0.2" date="2011-02-02" min="2.2.2">
			<![CDATA[
				Your release notes go in here. They will be parsed as Markdown.
				* So you can have a list
				* Or `code` if you like
			]]>
		</release>
		<release version="0.1" date="2011-01-01" min="2.0"/>
	</releases>

</extension>

extension.about.xsd

<?xml version="1.0" encoding="UTF-8"?>

<xs:schema
	xmlns:xs="http://www.w3.org/2001/XMLSchema"
	targetNamespace="http://symphony-cms.com/extensions/1.0"
	xmlns="http://symphony-cms.com/extensions/1.0"
	elementFormDefault="qualified">

	<!-- defines a symphony extension ID -->
	<xs:simpleType name="idType">
		<xs:restriction base="xs:string">
			<xs:pattern value="[a-z_]+" />
		</xs:restriction>
	</xs:simpleType>

	<!-- defines an http url -->
	<xs:simpleType name="httpType">
		<xs:restriction base="xs:string">
			<xs:pattern value="https?://(([a-z]+|[A-Z]+|[0-9]+|-+|_+)+\.)?([a-z]+|[A-Z]+|[0-9]+|-+|_+)+\.([a-z]+|[A-Z]+|[0-9]+|-+|_+)+(\.([a-z]+|[A-Z]+|[0-9]+|-+|_+)+)?((/([a-z]+|[A-Z]+|[0-9]+|-+|_+)+(/([a-z]+|[A-Z]+|[0-9]+|-+|_+)+)?)+)?" />
		</xs:restriction>
	</xs:simpleType>

	<!-- defines an email address -->
	<xs:simpleType name="emailType">
		<xs:restriction base="xs:string">
			<xs:pattern value="([a-z]|[A-Z]|[0-9]|-|_)+(([a-z]|[A-Z]|[0-9]|-|_|%)+)?@([a-z]|[A-Z]|[0-9]|-|_)+\.([a-z]|[A-Z]|[0-9]|-|_)+(\.([a-z]|[A-Z]|[0-9]|-|_)+)?" />
		</xs:restriction>
	</xs:simpleType>

	<!-- defines an accepted repository type -->
	<xs:simpleType name="repoType">
		<xs:restriction base="xs:string">
			<xs:pattern value="github" />
		</xs:restriction>
	</xs:simpleType>

	<!-- defines an accepted url type -->
	<xs:simpleType name="urlType">
		<xs:restriction base="xs:string">
			<xs:pattern value="homepage|wiki|other" />
		</xs:restriction>
	</xs:simpleType>

	<!-- defines a symphony/extension version number -->
	<xs:simpleType name="versionType">
		<xs:restriction base="xs:string">
			<xs:pattern value="\d\.\d(\.\d)?" />
		</xs:restriction>
	</xs:simpleType>

	<!-- defines an extension type keyword -->
	<xs:simpleType name="typeType">
		<xs:restriction base="xs:string">
			<xs:pattern value="Event|Field|Interface|Multimedia|Text Formatter|Translation|Workflow|Other" />
		</xs:restriction>
	</xs:simpleType>

	<!-- defines a text boolean type of yes/no -->
	<xs:simpleType name="boolType">
		<xs:restriction base="xs:string">
			<xs:pattern value="yes|no" />
		</xs:restriction>
	</xs:simpleType>

	<!-- defines an extension description -->
	<xs:complexType name="descriptionNode">
		<xs:simpleContent>
			<xs:extension base="xs:string">
				<xs:attribute name="lang" type="xs:string" use="optional" />
			</xs:extension>
		</xs:simpleContent>
	</xs:complexType>

	<!-- defines an extension repository -->
	<xs:complexType name="repoNode">
		<xs:simpleContent>
			<xs:extension base="httpType">
				<xs:attribute name="type" type="repoType" />
			</xs:extension>
		</xs:simpleContent>
	</xs:complexType>

	<!-- defines an extension resource url -->
	<xs:complexType name="urlNode">
		<xs:simpleContent>
			<xs:extension base="httpType">
				<xs:attribute name="type" type="urlType" />
			</xs:extension>
		</xs:simpleContent>
	</xs:complexType>

	<!-- defines a set of extension types -->
	<xs:complexType name="typesNode">
		<xs:sequence>
			<xs:element name="type" type="typeType" minOccurs="1" maxOccurs="unbounded" />
		</xs:sequence>
	</xs:complexType>

	<!-- defines an author name -->
	<xs:complexType name="authorNameNode">
		<xs:simpleContent>
			<xs:extension base="xs:string">
				<xs:attribute name="symphony" type="xs:string" use="required" />
				<xs:attribute name="github" type="xs:string" use="optional" />
				<xs:attribute name="twitter" type="xs:string" use="optional" />
			</xs:extension>
		</xs:simpleContent>
	</xs:complexType>

	<!-- defines an extension author -->
	<xs:complexType name="authorNode">
		<xs:sequence>
			<xs:element name="name" type="authorNameNode" minOccurs="1" maxOccurs="1" />
			<xs:element name="email" type="emailType" minOccurs="0" maxOccurs="1" />
			<xs:element name="website" type="httpType" minOccurs="0" maxOccurs="1" />
		</xs:sequence>
	</xs:complexType>

	<!-- defines a set of extension authors -->
	<xs:complexType name="authorsNode">
		<xs:sequence>
			<xs:element name="author" type="authorNode" minOccurs="1" maxOccurs="unbounded" />
		</xs:sequence>
	</xs:complexType>

	<!-- defines an extension dependency -->
	<xs:complexType name="dependencyNode">
		<xs:simpleContent>
			<xs:extension base="idType">
				<xs:attribute name="version" type="versionType" use="required" />
			</xs:extension>
		</xs:simpleContent>
	</xs:complexType>

	<!-- defines a set of extension dependencies -->
	<xs:complexType name="dependenciesNode">
		<xs:sequence>
			<xs:element name="dependency" type="dependencyNode" minOccurs="1" maxOccurs="unbounded" />
		</xs:sequence>
	</xs:complexType>

	<!-- defines an extension release -->
	<xs:complexType name="releaseNode">
		<xs:simpleContent>
			<xs:extension base="xs:string">
				<xs:attribute name="version" type="versionType" use="required" />
				<xs:attribute name="date" type="xs:date" use="required" />
				<xs:attribute name="min" type="versionType" use="required" />
				<xs:attribute name="max" type="versionType" use="optional" />
			</xs:extension>
		</xs:simpleContent>
	</xs:complexType>

	<!-- defines a list of extension releases -->
	<xs:complexType name="releasesNode">
		<xs:sequence>
			<xs:element name="release" type="releaseNode" minOccurs="1" maxOccurs="unbounded" />
		</xs:sequence>
	</xs:complexType>

	<!-- defines an extension -->
	<xs:complexType name="extensionNode">
		<xs:sequence>
			<xs:element name="name" type="xs:string" minOccurs="1" maxOccurs="1" />
			<xs:element name="description" type="descriptionNode" minOccurs="1" maxOccurs="unbounded" />
			<xs:element name="repo" type="repoNode" minOccurs="1" maxOccurs="1" />
			<xs:element name="url" type="urlNode" minOccurs="0" maxOccurs="unbounded" />
			<xs:element name="types" type="typesNode" minOccurs="0" maxOccurs="1" />
			<xs:element name="authors" type="authorsNode" minOccurs="1" maxOccurs="1" />
			<xs:element name="dependencies" type="dependenciesNode" minOccurs="0" maxOccurs="1" />
			<xs:element name="releases" type="releasesNode" minOccurs="1" maxOccurs="1" />
		</xs:sequence>
		<xs:attribute name="id" type="idType" use="required" />
	</xs:complexType>

	<!-- define the root node -->
	<xs:element name="extension" type="extensionNode" />

</xs:schema>

PHP Validator

<?php

class Validator
{
	public $dom;
	public $xsd;
	public $errors;

	public function __construct($xml=null, $xsd=null)
	{
		if(!is_null($xml) && !is_null($xsd))
		{
			$this->xsd = $xsd;

			$this->dom = new DOMDocument();

			$this->dom->load($xml);
		}
	}

	/**
	 * Validate an XML document against a schema file
	 */
	public function validate()
	{
		libxml_use_internal_errors(true);

		try
		{
			if(!$this->dom->schemaValidate($this->xsd))
			{
				$this->displayErrors();
			}
		}
		catch(Exception $ex)
		{
			$this->errors[] = array(
				'message' => $ex->getMessage()
			);
		}

		libxml_use_internal_errors(false);
	}

	/**
	 * XML Validation single error display function.
	 *
	 * @return Array    Error message array generated by failure to validate
	 */
	public function displayError($error)
	{
		$return = array();

		switch($error->level)
		{
			case LIBXML_ERR_WARNING:
				$return['level'] = 'Warning';
			break;
			case LIBXML_ERR_ERROR:
				$return['level'] = 'Error';
			break;
			case LIBXML_ERR_FATAL:
				$return['level'] = 'Fatal Error';
			break;
		}

		$return['code'] = $error->code;

		$return['message'] = trim($error->message);

		if($error->file)
		{
			$return['file'] = $error->file;
		}

		$return['line'] = $error->line;

		return $return;
	}

	/**
	 * XML Validation errors display function.
	 *
	 * @return String    Error message generated by failure to validate
	 */
	public function displayErrors()
	{
		$errors = libxml_get_errors();

		foreach($errors as $error)
		{
			$this->errors[] = $this->displayError($error);
		}

		var_dump($this->errors);

		libxml_clear_errors();
	}
}

$Validator = new Validator('extension.about.xml', 'extension.about.xsd');

$Validator->validate();

Needs testing, especially the regex

Updated the xml, xsd needs to reflect the change, after discussion

Completed!