Programming PHP

What you do with a processing instruction is up to you. One trick is to embed PHP
code in an XML document and, as you parse that document, execute the PHP code
with the eval( ) function. Example 11-5 does just that. Of course, you have to trust
the documents you’re processing if you eval( ) code in them. eval( ) will run any
code given to it—even code that destroys files or mails passwords to a hacker.
Example 11-5. Processing instruction handler
function processing_instruction($inParser, $inTarget, $inCode) {
if ($inTarget === 'php') {
eval($inCode);
}
}
Entity Handlers
Entities in XML are placeholders. XML provides five standard entities (&, >,
<, ", and '), but XML documents can define their own entities. Most
entity definitions do not trigger events, and the XML parser expands most entities in
documents before calling the other handlers.
Two types of entities, external and unparsed, have special support in PHP’s XML
library. An external entity is one whose replacement text is identified by a filename
or URL rather than explicitly given in the XML file. You can define a handler to be
called for occurrences of external entities in character data, but it’s up to you to parse
the contents of the file or URL yourself if that’s what you want.
An unparsed entity must be accompanied by a notation declaration, and while you
can define handlers for declarations of unparsed entities and notations, occurrences
of unparsed entities are deleted from the text before the character data handler is
called.
External entities
External entity references allow XML documents to include other XML documents.
Typically, an external entity reference handler opens the referenced file, parses the
file, and includes the results in the current document. Set the handler with xml_set_
external_entity_ref_handler( ), which takes in a reference to the XML parser and
the name of the handler function:
xml_set_external_entity_ref_handler(parser, handler);
The external entity reference handler takes five parameters: the parser triggering the
handler, the entity’s name, the base URI for resolving the identifier of the entity
(which is currently always empty), the system identifier (such as the filename), and
the public identifier for the entity, as defined in the entity’s declaration:
$ok = my_ext_entity_handler(parser, entity, base, system, public);
268 | Chapter 11: XML
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