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PHP XML expat parser

2022-06-29 02:32:00 【Crooning ~ singing】

XML What is it? ？

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

stay XML in , There are no predefined tags . You must define your own label .

To learn more about XML Knowledge , Please visit our  XML course .

Expat What is it? ？

To read and update - Create and process - One XML file , You need to XML Parser .

There are two basic XML Parser type ：

  • Tree based parser ： This parser puts XML Convert the document into a tree structure . It analyzes the entire document , It also provides access to the elements in the tree , For example, document object model (DOM).
  • Event based parser ： take XML Documents are treated as a series of events . When a specific event occurs , The parser will call functions to handle .

Expat A parser is an event based parser .

Event based parsers focus on XML The content of the document , Not their structure . Because of that , Event based parsers can access data faster than tree based parsers .

Please look at the following XML fragment ：

<from>Jani</from>

The event based parser puts the above XML Reported as a series of three events ：

  • Start element ：from
  • Start CDATA part , value ：Jani
  • Close element ：from

above XML Examples contain well formed XML. But this instance is invalid XML, Because there is no document type declaration associated with it (DTD).

However , In the use of Expat When parsing , It makes no difference .Expat It's a parser that doesn't check for validity , Ignore anything DTD.

As an event based 、 Not verified XML Parser ,Expat Fast and lightweight , Perfect for PHP Of Web Applications .

notes ：XML Documentation must be well formed , otherwise Expat Will generate an error .

install

XML Expat The parser function is PHP Core components . These functions can be used without installation .

XML file

Below XML The file will be applied 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>

initialization XML Parser

We will be having PHP In the initialization XML Parser , For different XML Event definition processor , 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 be output ：

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

working principle ：

  1. adopt xml_parser_create() Function initialization XML Parser
  2. Create functions that work with different event handlers
  3. add to xml_set_element_handler() Function to define , Which function is executed when the parser encounters start and end tags
  4. add to xml_set_character_data_handler() Function to define , Which function is executed when the parser encounters character data
  5. adopt xml_parse() Function to parse the file "test.xml"
  6. In case of mistakes , add to xml_error_string() Function XML Error converted to text description
  7. call xml_parser_free() Function to release the assigned to xml_parser_create() Function memory
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