XML (eXtensible Markup Language) is a data format for structured document interchange on the Web. It is a standard defined by The World Wide Web consortium (W3C). Information about XML and related technologies can be found at http://www.w3.org/XML/.
This PHP extension implements support for James Clark's expat in PHP. This toolkit lets you parse, but not validate, XML documents. It supports three source character encodings also provided by PHP: US-ASCII, ISO-8859-1 and UTF-8. UTF-16 is not supported.
This extension uses expat, which can be found at http://www.jclark.com/xml/. The Makefile that comes with expat does not build a library by default, you can use this make rule for that:
libexpat.a: $(OBJS) ar -rc $@ $(OBJS) ranlib $@
If you are using Apache-1.3.7 or later, you already have the required expat library. Simply configure PHP using --with-xml (without any additional path) and it will automatically use the expat library built into Apache.
On UNIX, run configure with the --with-xml option. The expat library should be installed somewhere your compiler can find it. If you compile PHP as a module for Apache 1.3.9 or later, PHP will automatically use the bundled expat library from Apache. You may need to set CPPFLAGS and LDFLAGS in your environment before running configure if you have installed expat somewhere exotic.
Build PHP. Tada! That should be it.
The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.
The XML event handlers defined are:
Table 1. Supported XML handlers
|PHP function to set handler||Event description|
|xml_set_element_handler()||Element events are issued whenever the XML parser encounters start or end tags. There are separate handlers for start tags and end tags.|
|xml_set_character_data_handler()||Character data is roughly all the non-markup contents of XML documents, including whitespace between tags. Note that the XML parser does not add or remove any whitespace, it is up to the application (you) to decide whether whitespace is significant.|
|xml_set_processing_instruction_handler()||PHP programmers should be familiar with processing instructions (PIs) already. <?php ?> is a processing instruction, where php is called the "PI target". The handling of these are application-specific, except that all PI targets starting with "XML" are reserved.|
|xml_set_default_handler()||What goes not to another handler goes to the default handler. You will get things like the XML and document type declarations in the default handler.|
|xml_set_unparsed_entity_decl_handler()||This handler will be called for declaration of an unparsed (NDATA) entity.|
|xml_set_notation_decl_handler()||This handler is called for declaration of a notation.|
|xml_set_external_entity_ref_handler()||This handler is called when the XML parser finds a reference to an external parsed general entity. This can be a reference to a file or URL, for example. See the external entity example for a demonstration.|
The element handler functions may get their element names case-folded. Case-folding is defined by the XML standard as "a process applied to a sequence of characters, in which those identified as non-uppercase are replaced by their uppercase equivalents". In other words, when it comes to XML, case-folding simply means uppercasing.
By default, all the element names that are passed to the handler functions are case-folded. This behaviour can be queried and controlled per XML parser with the xml_parser_get_option() and xml_parser_set_option() functions, respectively.
The following constants are defined for XML error codes (as returned by xml_parse()):
PHP's XML extension supports the Unicode character set through different character encodings. There are two types of character encodings, source encoding and target encoding. PHP's internal representation of the document is always encoded with UTF-8.
Source encoding is done when an XML document is parsed. Upon creating an XML parser, a source encoding can be specified (this encoding can not be changed later in the XML parser's lifetime). The supported source encodings are ISO-8859-1, US-ASCII and UTF-8. The former two are single-byte encodings, which means that each character is represented by a single byte. UTF-8 can encode characters composed by a variable number of bits (up to 21) in one to four bytes. The default source encoding used by PHP is ISO-8859-1.
Target encoding is done when PHP passes data to XML handler functions. When an XML parser is created, the target encoding is set to the same as the source encoding, but this may be changed at any point. The target encoding will affect character data as well as tag names and processing instruction targets.
If the XML parser encounters characters outside the range that its source encoding is capable of representing, it will return an error.
If PHP encounters characters in the parsed XML document that can not be represented in the chosen target encoding, the problem characters will be "demoted". Currently, this means that such characters are replaced by a question mark.
Here are some example PHP scripts parsing XML documents.
This first example displays the stucture of the start elements in a document with indentation.
Example 1. Show XML Element Structure
Example 2. Map XML to HTML
This example maps tags in an XML document directly to HTML tags. Elements not found in the "map array" are ignored. Of course, this example will only work with a specific XML document type.
This example highlights XML code. It illustrates how to use an external entity reference handler to include and parse other documents, as well as how PIs can be processed, and a way of determining "trust" for PIs containing code.
XML documents that can be used for this example are found below the example (xmltest.xml and xmltest2.xml.)
Example 3. External Entity Example
Example 4. xmltest.xml
This file is included from xmltest.xml: