PHP XML Expat

The built-in Expat parser makes it possible to process XML documents in PHP.

What is XML?

##XML is used to describe data, with the focus being on what the data is. XML files describe the structure of data.

In XML, there are no predefined tags. You must define your own tags.

To learn more about XML, visit our XML tutorial.

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What is Expat?

To read and update - create and process - an XML document, you need an XML parser.

There are two basic types of XML parsers:

· Tree-based parser: This parser converts the XML document into a tree structure. It analyzes the entire document and provides access to elements in the tree, such as the Document Object Model (DOM).

· Event-based parser: Treat XML documents as a series of events. When a specific event occurs, the parser calls a function to handle it.

The Expat parser is an event-based parser.

Event-based parsers focus on the content of XML documents, not their structure. Because of this, event-based parsers are able to access data faster than tree-based parsers.

Please see the following XML fragment:

##Jani

The event-based parser reports the above XML as a series of three events:

· Starting element: from

· Starting CDATA part, value: Jani

· Closing element: from

The XML example above contains well-formed XML. However, this instance is invalid XML because there is no document type declaration (DTD) associated with it.

However, this makes no difference when using the Expat parser. Expat is a parser that does not check validity and ignores any DTD.

As an event-based, non-validated XML parser, Expat is fast and lightweight, making it ideal for PHP web applications.

Note

: The XML document must be well formed, otherwise Expat will generate an error.

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InstallationThe XML Expat parser function is an integral part of PHP core. No installation is required to use these functions.

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XML fileThe following parser_expat.xml file will be used in our example:

  Tove Jani Reminder Don't forget me this weekend! 

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Initialize XML parser

We are going to initialize the XML parser in PHP, define handlers for different XML events, and then parse this XML file.

Example

"; break; case "TO": echo "To: "; break; case "FROM": echo "From: "; break; case "HEADING": echo "Heading: "; break; case "BODY": echo "Message: "; } } //Function to use at the end of an element function stop($parser,$element_name) { echo "
"; } //Function to use when finding character data function char($parser,$data) { echo $data; } //Specify element handler xml_set_element_handler($parser,"start","stop"); //Specify data handler xml_set_character_data_handler($parser,"char"); //Open XML file $fp=fopen("parser_expat.xml","r"); //Read data while ($data=fread($fp,4096)) { xml_parse($parser,$data,feof($fp)) or die (sprintf("XML Error: %s at line %d", xml_error_string(xml_get_error_code($parser)), xml_get_current_line_number($parser))); } //Free the XML parser xml_parser_free($parser); ?>

The above code will output:

-- Note --
To: Tove
From: Jani
Heading: Reminder
Message: Don't forget me this weekend!

##Working principle:

1. Initialize the XML parser through the xml_parser_create() function

2. Create functions that match different event handlers

3. Add the xml_set_element_handler() function to define, when Which function is executed when the parser encounters the start and end tags

4. Add the xml_set_character_data_handler() function to define which function is executed when the parser encounters character data

5. Through xml_parse( ) function to parse the file "test.xml"

6. In case there is an error, add the xml_error_string() function to convert the XML error into a text description

7. Call the xml_parser_free() function to Release the memory allocated to the xml_parser_create() function

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##More information about the PHP Expat parserFor more information about the PHP Expat function, please visit our PHP XML Parser Reference Manual.