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About the basics of PHP XML Expat parser

jacklove

Release： 2023-03-25 14:28:02

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PHP XML Expat parser plays an important role in PHP. This article will explain its related knowledge points in detail.

What is XML?

XML is used to describe data, and its focus is 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.

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 parsers: 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 will call function to handle it.

The Expat parser is an event-based parser.

Event-based parsers focus on the content of XML documents rather than their structure. Because of this, event-based parsers are able to access data faster than tree-based parsers.

Look at the following XML fragment:

Jani

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

Start 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 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.

Installation

The XML Expat parser function is an integral part of PHP core. 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&#39;t forget me this weekend!</body>
</note>

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Initializing the 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);
?>

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The above code will output:

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

Working principle:

Initialize the XML parser through the xml_parser_create() function

Create with different Event processingFunctions of the program

Add the xml_set_element_handler() function to define which function is executed when the parser encounters the start and end tags

Add the xml_set_character_data_handler() function to define when the parser encounters the start and end tags Which function is executed when character data is encountered

Parse the file "test.xml" through the xml_parse() function

In case there is an error, add the xml_error_string() function to convert the XML error into text Description

Call the xml_parser_free() function to release the memory allocated to the xml_parser_create() function

This article explains the basic knowledge points of the PHP XML Expat parser in detail, and more learning materials are clear Follow php Chinese website to watch.

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