PHP XML Parsing Complex XML Documents

PHP XML

Parsing Complex XML Documents

Problem

You have a complex XML document, such as one where you need to introspect the document to determine its schema, or you need to use more esoteric XML features, such as processing instructions or comments.

Solution

Use the DOM extension. It provides a complete interface to all aspects of the XML specification:

        // $node is the DOM parsed node <book cover="soft">PHP Cookbook</book>

        $type = $node->nodeType;

        switch($type) {

        case XML_ELEMENT_NODE:

               // I'm a tag. I have a tagname property.

               print $node->tagName; // prints the tagname property: "book"

               break;

        case XML_ATTRIBUTE_NODE:

               // I'm an attribute. I have a name and a value property.

               print $node->name; // prints the name property: "cover"

               print $node->value; // prints the value property: "soft"

               break;

        case XML_TEXT_NODE:

               // I'm a piece of text inside an element.

               // I have a name and a content property.

               print $node->nodeName; // prints the name property: "#text"

               print $node->nodeValue; // prints the text content: "PHP Cookbook"

               break;

       default:

               // another type

               break;

       }

       book

Discussion

The W3C’s DOM provides a platform- and language-neutral method that specifies the structure and content of a document. Using DOM, you can read an XML document into a tree of nodes and then maneuver through the tree to locate information about a particular element or elements that match your criteria. This is called tree-based parsing.

Additionally, you can modify the structure by creating, editing, and deleting nodes. In fact, you can use the DOM functions to author a new XML document from scratch.

One of the major advantages of DOM is that by following the W3C’s specification, many languages implement DOM functions in a similar manner. Therefore, the work of translating logic and instructions from one application to another is considerably simplified.

DOM is large and complex. For more information, read the specification or pick up a copy of XML in a Nutshell.

DOM functions in PHP are object oriented. To move from one node to another, access properties such as $node->childNodes, which contains an array of node objects, and $node->parentNode, which contains the parent node object. Therefore, to process a node, check its type and call a corresponding method, as shown:

         // $node is the DOM parsed node <book cover="soft">PHP Cookbook</book>

         $type = $node->nodeType;

         switch($type) {

         case XML_ELEMENT_NODE:

                // I'm a tag. I have a tagname property.

                print $node->tagName;  // prints the tagname property: "book"

                break;

         case XML_ATTRIBUTE_NODE:

                // I'm an attribute. I have a name and a value property.

                print $node->name;  // prints the name property: "cover"

                print $node->value;  // prints the value property: "soft"

                break;

         case XML_TEXT_NODE:

                // I'm a piece of text inside an element.

                // I have a name and a content property.

                print $node->nodeName;  // prints the name property: "#text"

                print $node->nodeValue;  // prints the text content: "PHP Cookbook"

                break;

         default:

                // another type

                break;

         }

To automatically search through a DOM tree for specific elements, use getElements ByTagname(). Here’s how to do so with multiple book records:

         <books>

                <book>

                       <title>PHP Cookbook</title>

                       <author>Sklar</author>

                       <author>Trachtenberg</author>

                       <subject>PHP</subject>

                </book>

                <book>

                       <title>Perl Cookbook</title>

                       <author>Christiansen</author>

                       <author>Torkington</author>

                       <subject>Perl</subject>

                </book>

         </books>

And to find all authors:

          // find and print all authors

          $authors = $dom->getElementsByTagname('author');

          // loop through author elements

          foreach ($authors as $author) {

                // childNodes holds the author values

                $text_nodes = $author->childNodes;

           

                foreach ($text_nodes as $text) {

                         print $text->nodeValue . "\n";

                }

          }

          Sklar

          Trachtenberg

          Christiansen

          Torkington

The getElementsByTagname() method returns an array of element node objects. By looping through each element’s children, you can get to the text node associated with that element. From there, you can pull out the node values, which in this case are the names of the book authors, such as Sklar and Trachtenberg.