Martijn's PHP Coding Blog

Below are some of the notes I made while preparing for the Zend PHP5 exam. They are a quick memory jog of how SimpleXML works and its limitations.

The SimpleXML extension provides easier access to XML files. It is simpler to use but also much more limited than the DOM XML extension. If your document makes use of XML namespaces (ns:foo) then it is better NOT to use SimpleXML as it will become anything but simple.

SimpleXML only supports XML 1.0. If fed an XML 1.1 document it will show “Unsupported version ‘1.1′” warnings, but will continue and attempt to read the file.

Removing elements is not really supported and the opportunities for modification are limited. SimpleXML is best used for straightforward reading and using of XML files.

1. Documentation

http://php.net/manual/en/book.simplexml.php

2. Demo XML file

This example XML catalog.xml file looks really familiar.

<?xml version="1.0"?>
<catalog>
    <book id="bk101">
        <author>Gambardella, Matthew</author>
        <title>XML Developer's Guide</title>
        <genre>Computer</genre>
        <price>44.95</price>
        <publish_date>2000-10-01</publish_date>
        <description>An in-depth look at creating applications with XML.</description>
    </book>
    <book id="bk102">
        <author>Ralls, Kim</author>
        <title>Midnight Rain</title>
        <genre>Fantasy</genre>
        <price>5.95</price>
        <publish_date>2000-12-16</publish_date>
        <description>A former architect battles corporate zombies,
         an evil sorceress, and her own childhood to become queen
         of the world.</description>
    </book>
</catalog>

3. Loading an XML file

There are four approaches to loading an XML file using SimpleXML.

3.1 Loading from a string

<?php
    $xml = file_get_contents("catalog.xml");
    $simple_xml_obj = simplexml_load_string($xml);
    if ($simple_xml_obj !== FALSE) {
        var_dump($simple_xml_obj);
    } else {
        echo "Invalid XML file" . PHP_EOL;
    }
?>

3.2 Loading the XML file directly

<?php
    $simple_xml_obj = simplexml_load_file("catalog.xml");
    if ($simple_xml_obj !== FALSE) {
        var_dump($simple_xml_obj);
    } else {
        echo "Invalid XML file" . PHP_EOL;
    }
?>

3.3 Loading from a string using the SimpleXML constructor

If creating an SimpleXML instance from a string we need to be wary of possible exceptions.

<?php
    $xml = file_get_contents("catalog.xml");
    try {
        $simple_xml_obj = new SimpleXMLElement($xml);
        var_dump($simple_xml_obj);
    }
    catch (Exception $e)  {
        echo "Oops: " . $e->getMessage() . PHP_EOL;
    }
?>

3.4 Loading the XML file directly with the class constructor

By setting the third parameter of the constructor (data_is_url) to true we can ask the class to directly load the XML file.

<?php
    $filename = "catalog.xml";
    try
    {
        $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);
        var_dump($simple_xml_obj);
    }
    catch (Exception $e)
    {
        echo "Oops: " . $e->getMessage() . PHP_EOL;
    }
?>

4. Usage examples

The benefit of SimpleXML over other solutions is that it makes accessing elements of the XML file EASY.

4.1 Stepping through the XML file by name

If you are familiar with the XML file and know the names of the elements you can refer directly to them in your code:

<?php
    $filename = "catalog.xml";
    $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);

    foreach($simple_xml_obj->book as $book)
    {
        // Access attributes through a named array
        echo "Book ID: {$book['id']}" . PHP_EOL;

        // Access sub elements using ->
        echo "Book Author: {$book->author}" . PHP_EOL;
        echo "Book Title: {$book->title}" . PHP_EOL;
        echo "Book Genre: {$book->genre}" . PHP_EOL;
        echo "Book Price: {$book->price}" . PHP_EOL;
        echo "Book Published date: {$book->publish_date}" . PHP_EOL;
        echo "Book Description: {$book->description}" . PHP_EOL;
    }
?>

4.2 Discovering the contents of the XML file

The children() method will returns an itterator of SimpleXMLElement elements containing each of the children. If used from the top node, this will allow you to step through the entire XML file. The attributes() method returns an iterator of key, value pairs containing all the attributes of the current XML tag.

<?php

    /* Discover the contents of an XML file recursivly
     *
     * @param    $input     A SimpleXMLElement to discover
     * @param    $depth     (optional) recursive dept, used for spacing
     * @returns
     */

    function DiscoverXML($xml,$depth = 0)
    {
        $spacing = str_repeat(' ',$depth);

        // Show the name of the current node
        echo $spacing . '<' . $xml->getName() . '>' ;

        // Show the value of the current node, do this by casting the
        // current element to a string.
        echo trim((string) $xml);

        // Show the attributes of the current node
        foreach($xml->attributes() as $name => $value)
            echo $spacing . " Atrribute {$name} = {$value}" . PHP_EOL;

        // Go deeper and discover any child nodes
        foreach($xml->children() as $child)
         DiscoverXML($child,$depth+1);

        // Close the current tag
        echo $spacing . '</' . $xml->getName() . '>' . PHP_EOL;
    }

    $filename = "catalog.xml";
    $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);

    DiscoverXML($simple_xml_obj);
?>

4.3 Changing elements

Existing elements can be simply modified by assigning them a new value.

<?php

    $filename = "catalog.xml";
    $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);

    $simple_xml_obj->book[0]->title = "Hello World";
    $simple_xml_obj->book[1]->title = "Vampire Mania";

    var_dump($simple_xml_obj);
?>

4.4 Adding a new element

<?php

    $filename = "catalog.xml";
    $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);

    $book = $simple_xml_obj->addChild('book');

    $book->addAttribute('id','1234');
    $book->addChild('author','Vars Unlimited');
    $book->addChild('title','PHP through the ages');
    $book->addChild('genre','historical');
    $book->addChild('price','10.95');
    $book->addChild('publish date','2010-1-1');
    $book->addChild('description','One of the finest examples of rewriting history');

    var_dump($simple_xml_obj);
?>

4.5 Using XPath

XPath, the XML Path Language, is a query language for selecting nodes from an XML document. In the code below we locate all the prices. Note the ‘element/sub element/sub sub element’ notation.

<?php

    $filename = "catalog.xml";
    $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);

    $search_result = $simple_xml_obj->xpath('/catalog/book/price');

    foreach($search_result as $price)
        echo $price . PHP_EOL;

?>

XPath is a query language — it allows you to select nodes based on criteria. For example, to only pick the books more expensive than $6 :

<?php

    $filename = "catalog.xml";
    $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);

    $search_result = $simple_xml_obj->xpath('/catalog/book[price>6]/price');

    foreach($search_result as $price)
        echo $price . PHP_EOL;

?>

4.6 Exporting XML

To export the in memory contents of a SimpleXMLElement we can use the ‘asXML’ method:

<?php
    $filename = "catalog.xml";
    $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);
    echo $simple_xml_obj->asXML();
?>

It is also possible to write the XML to disk by passing a file name:

<?php

    $filename = "catalog.xml";
    $simple_xml_obj = new SimpleXMLElement($filename,NULL,true);

    if (!$simple_xml_obj->asXML('/tmp/out.xml'))
        echo "Unable to save the XML file!" . PHP_EOL;

?>

Tags: php5, simplexml, zendexam

This entry was posted on Monday, November 9th, 2009 at 12:58 pm and is filed under PHP. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.