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PHP XML Expat Parser


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


What is XML?

XML is used to describe data, and its focus is on what the data is. The XML file describes the structure of the data.

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

To learn more about XML, please visit our XML Tutorial


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 transforms XML documents 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: Think of an XML document as a series of events. When a specific event occurs, the parser will call a function to handle it.

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 can access data faster than tree-based parsers.

Look at the following XML snippet:

<from>Jani</from>

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

  • Starting element: from
  • Start CDATA section, value: Jani
  • Close 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 for validity and ignores any DTDs.

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

Note: The XML document must be well-formed, or Expat will generate an error.


Installation

XML Expat parser functions are a core part of PHP. No installation is required to use these functions.


XML file

The following XML file will be used in our example:

<?xml version="1.0" encoding="ISO-8859-1"?>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>


Initialize XML parser

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

Example

<?php
//Initialize the XML parser
$parser=xml_parser_create();

//Function to use at the start of an element
function start($parser,$element_name,$element_attrs)
{
switch($element_name)
{
case "NOTE":
echo "-- Note --<br>";
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 "<br>";
}

//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("test.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!

How it works:

  1. Initialize the XML parser with the xml_parser_create () function
  2. Creating functions that work with different event handlers
  3. Add xml_set_element_handler () function to define which function to execute when the parser encounters the start and end tags
  4. Add xml_set_character_data_handler () function to define which function to execute when the parser encounters character data
  5. Parse the file "test.xml" with the xml_parse () function
  6. In case of errors, add xml_error_string () function to convert XML errors into text descriptions
  7. Call the xml_parser_free () function to free the memory allocated to the xml_parser_create () function