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XML Transformer Tutorial
By Kristian K�hntopp and Sebastian Bergmann.

0. This file

This file is a supplementary document for the online documentation of the
XML_Transformer PEAR package. It is not a comprehensive manual of methods and
parameters, that's what the PEAR online documentation is good for.

Instead, this document acts as a guide and tutorial to XML_Transformer and
friends. It aims at explaining the architecture of XML_Transformer and the
choices that governed its design. Also, it should contain a number of simple
applications of XML_Transformer to illustrate its typical use.

1. What is it good for?

The XML Transformer is a system of PEAR classes that can be used to transform
XML files into other XML files. The transformation is specified using PHP
functions.

We created XML Transformer because we were annoyed with the syntax and
capabilities of XSLT. XSLT is a very verbose language that needs many lines of
text to express even the simplest of algorithms. Also, XSLT is a functional
language offering all the drawbacks of languages of this class (variables are
actually a kind of constant, recursion is needed to express many loops etc)
without the advantages that come with such languages (closures, functions as
first-order datatypes etc). Finally, XSLT is badly integrated into almost all
development environments, offering little in the way of character manipulation,
and nothing in the way to database access, image manipulation, flat file output
control and so on.

XML Transformer can do many things that linear (non-reordering) XSLT can. It can
do some things XSLT can't (such as recursively reparsing its own output), and it
can utilize all PHP built-in functions and classes to do this. Transformations
are specified using the syntax of PHP with which you're already familiar, and
there is a simplified syntax to specify simple replacement transformations that
does not even need PHP at all.

Since XML Transformer uses a SAX parser to do its work, it can't do anything a
SAX parser can't do. That is, it cannot do reordering transformations in a
single pass. You won't be able to generate indices, tables of contents and other
summary operations in a single pass. If you run into such problems, think LaTeX
and use the solutions LaTeX uses for this problems as well - we have recently
added support for multipassing, so that implementing such a mechanism shouldn't
be too difficult. Also, we are providing the Docbook Namespace Handler as an
example for such mechanisms.

Finally, we are considering an implementation of XML Transformer using a DOM
parser and XPath queries to enable single pass reordering operations in PHP as
well.

2. What are all these files and classes?

2.1 XML_Transformer

The heart of the XML Transformer is defined in XML/Transformer.php. All the work
is being done within the transform() method, which takes an XML string,
transforms it and returns the transformed result.

As transform() uses PHP's XML extension internally, the XML string must in
theory be a well-formed XML fragment in order for the transformation to work.
That is, it should be starting with a tag and ending with the same tag. For
your convenience we internally wrap everything that is being transformed
into a <_>...</_> container in order to satisfy this requirement.

To set up a transformation, you need to create an instance of the class
XML_Transformer and then add options and transformations to it.

$t = new XML_Transformer();
  $t->setDebug(true);
  $t->overloadNamespace('php', new XML_Transformer_PHP);

Options are added using the set-type methods setDebug(),
setRecursiveOperation(), and setCaseFolding(). Transformations are added using
overloadNamespace(). All of these options and then some can be set as parameters
to the constructor. You'd be using an array that is being passed to the c'tor
for this.

$t = new XML_Transformer(
    'debug'                => true,
    'overloadedNamespaces' => array(
      'php',
      new XML_Transformer_PHP
    )
  );

2.2 XML_Transformer_CallbackRegistry and XML_Util

Internally, XML Transformer uses two auxiliary classes to do its work. One of
them is the XML_Transformer_CallbackRegistry, which does all the bookkeeping for
XML_Transformer, tracking which methods are to call for which namespace and so
on. XML_Transformer_CallbackRegistry is a Singleton, and the instance is
maintained automatically by XML_Transformer. You never use it directly.

The other used to be XML_Transformer_Util, which was later merged with other
methods and is now XML_Util, a PEAR package in its own right. Please refer
to the XML_Util documentation for more information on this class.

2.3 XML_Transformer_Namespace

Using XML_Transformer, all transformations are specified for namespaces. You may
specifiy transformations for the empty namespace, that is, you may transform
simple tags such as <body/> or . The name of the empty namespace is '' or
'&MAIN'.

To make the definition of namespaces easy, we supply a class
XML_Transformer_Namespace from which you can inherit (Note that
XML_Transformer_namespace is only one possible implementation for a namespace.
You are free to choose a different implementation schema anytime, for example if
the direct mapping of classes to namespaces is not applicable for your
deployment scenario). The class is suitable for all non-nesting tags and the
implementation schemata shown here are suitable for non-nesting tags such as
<img/> or <h1/>, but you'd need something more sophisticated to implement a
nesting structure such as <table/> which can contain itself.

In order to define a tag called <tag />, you create a class and implement
methods called start_tag($attributes) and end_tag($cdata). These methods must
return the result of the transformation as strings, and it must be a valid XML
fragment. By our coding conventions, start_tag() never returns anything but
only records the tags attributes. All code is being generated in end_tag().
That way we avoid problems with invalid XML in recursive parsing.

class MyNamespace extends XML_Transformer_Namespace {
  var $tag_attributes = array();

function start_tag($att) {
    $this->tag_attributes = $att;

return '';
  }

function end_tag($cdata) {
 if (isset($this->tag_attributes['name'])) {
 $name = $this->tag_attributes['name'];
 $thline = "<tr><th>$name</th></tr>";
 } else {
 $thline = '';
 }

return "<table>$thline<tr><td>$cdata</td></tr></table>";
 }
}

This minimal sample implements a container tag called <...:tag name="headline"
/>, which places its content in a table, and additionally supplied a table
headline in a <th/> cell if an attribute "name" is present.

The example is pretty much useless, but illustrates attribute capture, access to
the tags cdata content, and returning of results. Also, it illustrates how easy
namespaces are created by inheriting from XML_Transformer_Namespace.

To activate the namespace and assign it a namespace prefix, you'd use
overloadNamespace():

$t = new XML_Transformer(...);
  $t->overloadNamespace('my', new MyNamespace());

This tag can now be used as "<my:tag name='heading'>content</my:tag>".

The XML_Transformer_Namespace class has a few instance variables which may come
in handy in some cases. One of them is _transformer, which is indeed a reference
to the owning transformer.

Another is an array _prefix, which is an enumeration of namespace prefixes of
this namespace class. In our example above, that array would have just one
element, $this->_prefix[0], and it would contain the string 'my'. As you might
have guessed from the fact that _prefix is an array, we consider it legal to
register a single namespace class under multiple prefixes, if you can manage to
keep your references straight and not inadvertantly copy your instance. We have
not bothered to implement namespace scopes, though, as we should have were we in
the business of implementing the complete XML specification.

The XML_Transformer has a handy feature where Namespaces are autoloaded and
registered under their default namespace names, if they define one. In order for
autoloading to work, define an instance variable defaultNamespacePrefix as a
string. This string is the prefix under which the namespace will register itself
when autoloading.

Finally, a namespace may indicate that it requires two passes in order to
generate indicies or other data collections. If this is needed, the namespace
should set secondPassRequired to true (default: false).

2.3.1 Using autoloading

We have supplied a number of subclasses to XML_Transformer_Namespace. These
reside in a directory "./Transformer/Namespace" relative to the directory of the
actual Transformer.php file itself, and can be autoloaded.

In order to autoload namespaces, supply the flag "autoload" to your transformer
constructor. You may set the flag simply to "true" in order to load all
Namespaces, or you may pass a single string or an array of strings indicating
the namespaces you want to load.

Namespaces are connected to their default prefixes, and in order for this to
work they must define such prefixes in defaultNamespacePrefix.

Example:

$t = new XML_Transformer(
            array(
              'autoload' => true
            )
          );
 Load all Namespaces

$t = new XML_Transformer(
            array(
              'autoload' => 'PHP'
            )
          );
 Load XML/Transformer/Namespace/PHP.php.

$t = new XML_Transformer(
            array(
              'autoload' => array('PHP', 'Image', 'Anchor')
            )
          );
 Load the indicated namespaces.

Limitations of autoloading:

- currently, there is no pathname support. Only classes in
  "./Transformer/Namespace" can be autoloaded. Your project directories are
  not searched.

- currently, there is no separate method to trigger autoload.
  You must specify autoloading as a flag to the constructor.

2.4 supplied XML_Transformer Namespaces

All namespaces we supply are derived from XML_Transformer_Namespace and subject
to the limitations and interfaces of this baseclass.

If you are looking into our code in order to write your own namespaces, we
recommend you look into Anchor first. Anchor is your plain vanilla namespace
with no tricks and extra features.

The DocBook namespace is an example of a two-pass namespace. If you have an
application that needs to generate tables of contents, cross references or other
stuff that cannot be done without reordering, you should read DocBook as an
example.

The Image namespace generates PNG images, and uses a local cache for this. That
is, we generate files in our cache, and generate <img /> HTML tags that
references these files. This is fast, and saves us multiple renderings of the
same image. You should look into these techniques if your tags are graphically
intensive or otherwise ressource consuming. Also, <img:gtext /> uses a lot of
parameters, and often these are similar across multiple calls of <img:gtext />
on the same page. We supply <img:gtextdefault /> as a mechanism to provide
sensible defaults to subsequent calls. Look into our code to learn how we did
this.

The PHP namespace implements <php:define />, which gobbles up its contents
unparsed. In order to do this it uses getLock() and releaseLock() in
transformer. If you need code that is read as-is and evaluated later, look into
this. Also, the PHP namespace uses PHP's eval function extensively to generate
namespace classes at run-time. This is not a recommended practice, but probably
interesting code.

2.4.1 XML_Transformer_Namespace_Anchor

The Anchor namespace implements a number of tags that create indirect named
links (URNs): The link is specified by name, and the actual link location and
title are supplied from a database internal to the class. Additionally, a tag
that selects a random link is supplied.

The default namespace prefix for this Namespace is "a".

2.4.1.1 Link database

The link database is maintained internally as an array, _anchorDatabase and is
accessible through the setDatabase($db) and $db = getDatabase() accessor
methods.

$a = new XML_Transformer_Namespace_Anchor();
  $t = new XML_Transformer();
  $t->overloadNamespace('a', $a);

$a->setDatabase(
        array(
          'php'  => array(
                      'href'  => 'http://www.php.net',
                      'title' => 'PHP Homepage'
                    ),
          'pear' => array(
                      'href'  => 'http://pear.php.net',
                      'title' => 'PEAR Homepage'
                    )
        )
      );

Additionally, items may be added to or dropped from the database using the
addItem() and dropItem() methods. Also, individual items can be queried with
getItem().

$a->addItem(
    'dclpfaq',
    array(
      'href'  => 'http://www.dclp-faq.de',
      'title' => 'de.comp.lang.php FAQ Homepage'
    )
  );

$dclpfaq = $a->getItem('dclpfaq');
  echo $dclpfaq['href'];

$a->dropItem('dclpfaq');

Note that neither database nor the tags place any restrictions on the number or
kind of attributes ("href", "title", ...) stored in the database. All attributes
will be reproduced "as is" on the generated links.

2.4.1.2 The <a:iref> tag

The <a:iref iref="name" /> container will look the given name up in the database
and produce a HTML <a /> container. The attributes find in the link database
will be produced literally as attributes to the <a /> container and the contents
of the <a:iref /> will become the contents of the <a /> container.

Example:

<a:iref iref="php">The PHP Homepage</a:iref>

Result:

<a href="http://www.php.net" title="PHP Homepage">The PHP
Homepage</a>

2.4.1.3 The <a:random> tag

The <a:random /> container will select a random name from the database and link
to it. The contents of the <a:random /> become the contents of the generated
<a /> container.

Example:

<a:random>A random link</a:random>

Result:

<a href="http://www.php.net" title="PHP Homepage">A random link</a>

2.4.1.4 The <a:link> tag

The link tag will add a link to the database, and vanishes (generates no
output). The name attribute to link will define the link name, the other
attributes are copied into the database literally.

Example:

<a:link name="php"
 href="http://www.php.net"
 title="PHP Homepage" />

Result:

The link is added to the database. No output is being generated.

2.4.2 XML_Transformer_Namespace_DocBook

* TODO (sb)

2.4.3 XML_Transformer_Namespace_Image

The Image namespace implements a number of tags that are loosely related to
images. At the moment there is a tag that autogenerates height/width attributes
and another tag that turns its content text into a PNG with that text.

The default namespace prefix for this namespace is "img".

2.4.3.1 The <img> tag.

This tag will generate a <img /> tag with the original attributes, and will add
width and height attributes if possible.

Example:

<img:img src="somepng.png" alt="blah" />

Result:

2.4.3.2 The <gtext> tag

Gtext is short for graphical text. The tag will take its content and turn it
into a single PNG or a series of PNGs using ImageTTFText() internally.

Gtext takes a very large number of attributes. All of them can be specified as
defaults with <img:gtextdefault /> (see below) and individually overridden.

Limitations:

Gtext requires a directory called "/cache/gtext" below DocumentRoot that is
writeable by the webserver. Also, Fonts are looked for in
/usr/X11R6/lib/X11/fonts/truetype. Currently there is no API to override this.

Attributes:

- split

Split is either "none", "word" or "char". If it is "none", the complete
  content of a gtext is set as a single image. If it is "word", each word of the
  content is set as a separate image. This does not look as good as a single
  image, but can be word wrapped. If it is "char", each character is rendered as
  a single image. This loads very fast (multiple occurences of the same
  character are mapped onto the same file), but does not look right.

- font

Name of the TTF font to use.

If an absolute pathname is supplied, that font is being used. Otherwise
  /usr/X11R6/lib/X11/fonts/truetype/ is prepended to the font name and that file
  is tried.

- fontsize

What size to render the font in (TTF points).

- alt

By default, each generated image is created with the text on the image as alt
  Tag. This can be overridden by specifying an alt tag (not recommended).

Note that XML attributes may not contain tags. Thus, all markup is being
  stripped from the automatically generated alt tag in order to ensure well-
  formed HTML.

- bgcolor, fgcolor

The generated image is initially filled with bgcolor. This color is then set
  to transparent (unless prohibited, see below). After that, text is rendered in
  fgcolor into this image.

- antialias

The rendered text is by default generated with antialiasing. If you do not
  want the text to be antialiased, set the antialias attribute to "no".

- transparency

The background color is by default set to transparent. If you do not want the
  bgcolor to be set transparent, set the transparency attribute to "no".

- cacheable

By default, the generated image is stored in the gtext cache. After that,
  subsequent renderings of that image are not done. Instead the cached image is
  referenced.

If you set the cacheable attribute to "no", the image is recreated on each
 <img:gtext /> call. This is recommended during development.

- spacing

<img:gtext /> tries to create the generated PNG as small as possible. If you
 specify a spacing attribute, a transparent border of x pixels is added around
 all four borders of the generated image.

- border

Additionally, if you specify a border, an x pixel border is added around the
  text and the spacing. Unlike spacing the border is painted in bordercolor.

- bordercolor

The color to paint the border in is specified using the
  bordercolor attribute.

Example:

<img:gtextdefault bgcolor="888888" fgcolor="#000000"
 font="arial.ttf" fontsize="33"
 border="2" spacing="2"
 split="" cacheable="no"/>

<img:gtext>Antialias</img:gtext> 
 <img:gtext antialias="no">No Antialias</img:gtext> 
 <img:gtext>OoAaMmXxGgQqpP�</img:gtext> 
 <img:gtext transparency="no">abcdefghijklmnopqrstuvwxyz</img:gtext> 
 <img:gtext>ABCDEFGHIJKLMNOPQRSTUVWXYZ</img:gtext> 
 <img:gtext>0123456789 �������</img:gtext> 
 <img:gtext>!�$%/()=?+*~#',;.:-_|</img:gtext> 
 <img:gtext split="word">Ein Satz mit x.</img:gtext> 
 <img:gtext split="char">Alphabet Soup!</img:gtext>

2.4.3.3 The <gtextdefault> tag

<img:gtextdefault /> takes all attributes of <img:gtext /> and stores them.
These attributes are then supplied to all subsequent <img:gtext /> calls. They
may be overriden by specifiying alternative values in the <img:gtext />.

<img:gtextdefault /> is empty and renders to nothing.

Example:

See above, <img:gtext /> for an example.

2.4.4 XML_Transformer_Namespace_PHP

The PHP namespace allows you to define namespaces on the fly, to access PHP
internal variables in a general manner from XML code. It also allows you to
embedPHP code into your XMLcode, which to avoid XML_Transformer was written in
thefirst place. Thus, the PHP namespace is evil. Do not use it.

2.4.4.1 The <namespace> and <define> tags

In order to define a new namespace you need to generate a subclass of the
XML_Transformer_Namespace class and write start and end functions for all
tags that should be in that namespace. Because mouseprodding HTML designers
cannot be expected to touch PHP code, the <namespace> tag creates a new
XML_Transformer_Namespace subclass and the <define> tag allows you to
define new tag processing functions inside that namespace.

These tag processing functions are somewhat limited in their functionality,
though: We simply record the XML inside the <define> tag and have the function
output that code as a replacement for the defined tag.

Within the replacement, the tags content may be accessed as $content, and
its attributes may be accessed as $-variables as well.

Example:

<php:namespace name="define">
 <php:define name="test">
 <img:gtext>The content is $content, and attribute x is $x.</img:gtext>
 </php:define>

<php:define name="case">
 Even more $content
 </php:define>
 </php:namespace>

Defines a namespace "define" with the tags "test" and "case".

You can now write

<define:test x='y'>Blah</define:test>

yielding

<img:gtest>The content is Blah, and attribute x is y.</img:gtext>

which will then be recursively reparsed into a PNG image containing this
text.

You can also use

<define:case>Content!</define:case>

yielding

Even more Content!

These two tags have interesting source code.

2.4.4.2 The <expr> and <logic> tags

These tags evaluate their content as a php expression or code block
and return the result.

Thus, <php:expr>3+3</php:expr> results in the code 'return 3+3' being
evaluated, resulting in the tag sequence being replaced by the text "6".

Likewise, <php:logic>echo "Hello, world"</php:logic> results in
thatcode being evaluated and sequence being replaced by the codes output.

Please note that <php:logic/> make use of the output buffering functions
internally and does not work at all if you are using the
XML_Transformer_OutputBuffer driver. This is due to a bug in PHP 4.2.3.

Please note as well that using <expr/> and <logic/> is bad design and
strongsly discouraged. You should have been writing custom tags for this
in the first place.

2.4.4.3 <get>, <post>, <cookie>, <request>,
 <server>, <session>, <variable>

These tags all accept a single attribute 'name'. They will return the value
of a PHP variable of that name from their respective namespace.

<php:get name='a'/> will return $_GET['a'].
<php:post name='a'/> will return $_POST['a'].
<php:cookie name='a'/> will return $_COOKIE['a'].
<php:request name='a'/> will return $_REQUEST['a'].

<php:server name='a'/> will return $_SERVER['a'].
<php:session name='a'/> will return $_SESSION['a'].
<php:variable name='a'/> will return $GLOBALS['a'].

2.4.4.4 <setvariable>

This tag will assign its contents to the global variable named in the 'name'
attribute.

Example:
<php:setvariable name='a'>10</php:setvariable>

This executes $GLOBALS['a'] = 10.

2.5 Output Drivers

XML_Transformer is designed to be used as a bare class by calling the
transform() method. That call can be easily wrapped into output buffer
handlers, caches or other more complicated setups.

We provide two standard setups for XML_Transformer as subclasses to the
transformer class: One using PEAR's Cache_Lite to cache transformation
results, and one using PHP's output buffering functions with a callback
to transform XML on the fly.

2.5.1 XML_Transformer_Driver_Cache

This subclass of XML_Transformer requires PHP's Cache_Lite to be installed.
All parameters of the constructor are passed to XML_Transformer as well
as to Cache_Lite, their parameter names magically being nonoverlapping.

It overrides XML_Transformer's transform() method with

function transform($xml, $cacheID = '')

The cacheID is a unique identifier for the $xml string that is to be
transformed. It may be the md5 hash of the name and date of the file
that provided the $xml string. If no cacheID is being provided, the
method uses md5($xml) internally as a cacheID.

The method will look up content for this cacheID, and if there is none,
will perform the transformation and save the result under this cacheID.

Normal transformation results are being returned.

2.5.2 XML_Transformer_Driver_OutputBuffer

This subclass of XML_Transformer uses PHP's Output Buffering mechanism to catch
theoutput of a script, transforms it, and outputs the
result.

Example:
 <?php

require_once 'XML/Transformer/Driver/OutputBuffer.php';
    require_once 'XML/Transformer/Namespace.php';

class Main extends XML_Transformer_Namespace {
 function start_bold($attributes) {
 return '';
 }

function end_bold($cdata) {
 return $cdata . '';
 }
 }

$t = new XML_Transformer_Driver_OutputBuffer(
 array(
 'overloadedNamespaces' => array(
 '&MAIN' => new Main
 )
 )
 );
 ?>
 <bold>text</bold>

Output

text

Normally, you'd have all the PHP code in an auto_prepend_file and store the 
plainXHTML (here: text) in .html files. Then you map PHP so that .html
files arebeing processed. Magically all the XML in there is being transformed.

3. Debugging

* TODO entire section

3.1 The debugging filter

3.2 Debugging recursion

3.3 Debugging and the output buffer

4. Caching and XML_Transformer

* TODO (kk) entire section

4.1 Adressable and hidden caches

* Images and server fast path, cache must be below document root
* generated HTML and templates need not be addressable, outside
  document root

4.2 What caching is about

* Caching means NOT to work
* Cache keys determine the cached object,
  cache key must include all items that can cause the
  cached object to vary
* Cache keys should not be the cached object, that is,
  $key = md5($transformation_result) is useless.
* Do we need to clean out the cache?

* Caching fragments

4.3 How XML_Transformer_Namespace_Image uses caching

4.4 How XML_Transformer_Cache uses caching

4.5 How to deploy caching manually