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 data is. An 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 ourXML 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 type of parser converts an XML document into a tree structure. It analyzes the entire document and provides access to the elements in the tree, such as the Document Object Model (DOM).
- Event-based parsers: They treat an XML document 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 the XML document, not their structure. Because of this, event-based parsers can access data faster than tree-based parsers.
Look at the following XML fragment:
The event-based parser reports the XML above 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 example is invalid XML because there is no Document Type Declaration (DTD) associated with it.
However, when using the Expat parser, this makes no difference. Expat is a non-validating parser that ignores any DTD.
As an event-based, non-validating XML parser, Expat is fast and lightweight, making it very suitable for PHP web applications.
Note:XML documents must be well-formed, otherwise Expat will generate an error.
Installation
The XML Expat parser functions are part of the PHP core. You can use these functions without installation.
XML File
The following XML file will be used in our example:
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
Initialize XML Parser
We will initialize the XML parser in PHP, define handlers for different XML events, and then parse the XML file.
Example
//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:
To: Tove
From: Jani
Heading: Reminder
Message: Don't forget me this weekend!
How it works:
- Initialize the XML parser with the xml_parser_create() function
- Create functions to work with different event handlers
- Add the xml_set_element_handler() function to define which function to execute when the parser encounters start and end tags
- Add the xml_set_character_data_handler() function to define which function to execute when the parser encounters character data
- Parse the file "test.xml" with the xml_parse() function
- In case of an error, add the xml_error_string() function to convert an XML error into a text description
- Call the xml_parser_free() function to release the memory allocated by the xml_parser_create() function
More information about PHP Expat Parser
For more information about PHP Expat functions, please visit ourPHP XML Parser Reference Manual。