Getting started with Xpath

Published May 7, 2015

The content here is under the Attribution 4.0 International (CC BY 4.0) license

It’s been a long time since my last post and now I have come up with a new subject interesting to me because I could find around me people that know and understand Xpath, so here I’m going to show what I’ve found about Xpath and the little particularities on it in PHP.

A little bit of history

The first version (1.0) of Xpath was released in 1999 and the second version (2.0) came in 2011 nowadays we have the Xpath 3.0 released in 2014. Xpath was created to be able to easily navigate between nodes in an XML document and be able to find specific nodes as well by criteria using a query language.

Xpath syntax

The Xpath became famous for its light syntax and facilities for manipulating nodes on XML documents. Xpath syntax queries are alike URLs, for example, you can use a common XPath such as “/rootNode/child/lastchild” to refer to the last child directly, let’s see the example below and make things cleaner.

<?xml version="1.0" encoding="utf-8"?>
<library>
  <book id="1">
    <author>Author 1</author>
    <summary>Summary 1</summary>
  </book>
  <book id="2">
    <author>Author 2</author>
    <summary>Summary 2</summary>
  </book>
  <book id="3">
    <author>Author 3</author>
    <summary>Summary 3</summary>
  </book>
</library>

Following our thoughts to reach the tag we can do "/library/book" so this way we are going to have three elements. We can also be more specific with our document, let's say that we want to have just the book with the id 1, this way we can use "/library/book[@id=1]"

tip: Google for xpath tester and try out the examples in this post

Expression	Description
/	Selects from the root node
//	Selects nodes in the document from the current node that
match the selection no matter where they are
.	Selects the current node
..	Selects the parent of the current node
@	Selects attributes

Example from w3schools: w3schools

Xpath and PHP

Now we have the basics of Xpath we can play around with PHP and see how it interacts with Xpath. The first thing we must have in mind is that PHP has two alternatives to accomplish this task and one is using simplexml family functions or we have also the DOMXpath family.

<?php
$xml = simplexml_load_file('library.xml');

$xml->xpath('/library/book[@id=1]');

Using simplexml the function will return to us all matched elements inside an array with SimpleXMLElement on it.

Array
(
    [0] => SimpleXMLElement Object
        (
            [@attributes] => Array
                (
                    [id] => 1
                )

            [author] => Author 1
            [summary] => Summary 1
        )
)

Otherwise, we can use the DOMXpath to complete our task but we will need a few more lines. This approach uses an Object Oriented Programming approach.

<?php
$dom = new DOMDocument();
$dom->load('library.xml');

$xpath = new DOMXpath($dom);

Therefore the return change as well

DOMNodeList Object
(
    [length] => 1
)

Try it yourself

We have many possibilities when we are using Xpath and also we can do a lot of things with it, the only way to get know is by getting your hands dirty and trying different queries in different files!