 | Level: Intermediate Vikram Vaswani, Founder, Melonfire
25 Mar 2008 Trackbacks play an important role in allowing blog readers to find and follow links to useful related content. The PEAR Services_Trackback package lets developers manually generate these trackbacks through a well-defined PHP API that takes care of creating trackback requests and parsing XML responses.
Introduction
One of the nice things about the blogosphere is how highly-networked and rapidly-evolving a community it is. By posting useful content and linking it to relevant material posted by other bloggers, an individual blogger can rapidly make his presence felt in the community and even build a fan base in a fraction of the time it would take in the real world.
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Frequently used acronyms
- API - application programming interface
- PHP - PHP: Hypertext Preprocessor
- XML - Extensible Markup Language
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Among the various tools used by bloggers to link posts together is the
trackback, which provides a mechanism for one blogger to tell another of a related post or resource through a backlink. Trackbacks play an important role in strengthening the inter-connections between blogs, and also provide a convenient (and automatic) way for blog readers to find out about useful, related content.
If you already have a blog, chances are that you can use the built-in features
of your blog to create trackbacks to content on other blogs. But what if you
don't have a blog, but still want to create trackbacks between your Web posts
and other blogs? Well, if you use PHP, you can benefit from the
Services_Trackback package, which provides a generic mechanism to manually create and send trackbacks.
Installation
The Services_Trackback package is maintained by Tobias Schlitt, and released to the PHP community under a PHP license. It is freely available from PEAR, the PHP Extension and Application Repository, and the easiest way to install it is with the automated PEAR installer, which should have been included by default with your PHP build. To install it, simply issue the following command at your shell prompt:
shell> pear install Services_Trackback
The PEAR installer will now connect to the PEAR package server, download the package, and install it to the appropriate location on your system. This tip uses Services_Trackback V0.6.1 (beta).
To install the package by hand, visit its home page, download the source code
archive, and manually uncompress the files to the desired location. Note that this manual installation process presupposes some knowledge of PEAR's package organization structure. You'll find a link to the package's home page in the Resources of this tip.
Overview and Basic Usage
Before you dive into Services_Trackback, a few words about how trackbacks work
are in order. The original Trackback specification was developed by SixApart,
and it specifies that a trackback request is initiated by sending a POST request
over HTTP to a specified trackback URL. This trackback URL is the URL defined by the source blog to receive trackback requests for a specific post.
This POST request contains various bits of information to make the trackback
more useful:
- The title and name of the Web page initiating the trackback
- A relevant excerpt from the page content
- The URL of the page that initiates the trackback
Of these pieces of information, only the URL of the page that initiates the trackback
is mandatory; however, to create trackbacks that are relevant and useful, you should always include meaningful content for the other fields as well.
Listing 1 has an example of one such POST request, which creates a trackback to the original post at http://www.originating.blog.com/the/original/post:
Listing 1: A POST request for a trackback
POST /the/original/post/trackback
Content-Type: application/x-www-form-urlencoded
title=Trackback+Test&url= http%3A%2F%2Fmy.blog.domain%2Fsome%2Fpost%2F
&excerpt=This+is+what+I+said+about+your+post...&blog_name=My+Blog
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Now, when the destination server receives this trackback request, the server generates one of two responses, both formatted in XML. If the trackback request is in order, does not appear to be spam and the host initiating the trackback does not appear on a blacklist, the server will typically accept and create the trackback link, responding with the success message in Listing 2:
Listing 2: An XML success response
<?xml version="1.0" encoding="utf-8"?>
<response>
<error>0</error>
</response>
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If, on the other hand, the trackback fails, an error message like Listing 3 is generated:
Listing 3: An XML error response
<?xml version="1.0" encoding="utf-8"?>
<response>
<error>11</error>
<message>Database error, could not save trackback</message>
</response
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Fortunately, Services_Trackback does all the work to generate the POST request
and decode the response for you. Listing 4 demonstrates typical usage of this package:
Listing 4: Generating a trackback with PHP
<?php
$trackbackData = array(
'title' => 'Trackback Test',
'excerpt' => 'This is what I said about your post...',
'url' => 'http://my.blog.domain/some/post/',
'blog_name' => 'My Blog',
'trackback_url' => 'http://www.originating.blog.com/the/original/post/trackback'
);
// include class
include "Services/Trackback.php";
// initialize new instance
$trackback = new Services_Trackback();
// set object properties
foreach ($trackbackData as $k => $v) {
$trackback->set($k, $v);
}
// send trackback
$ret = $trackback->send();
if (PEAR::isError($ret)) {
echo "ERROR. Trackback failed because: " . $ret->getMessage();
} else {
echo "Trackback successfully posted";
}
?>
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Listing 4 uses Services_Trackback package in PHP to create
and post a trackback to a blog post. It begins by reading the Services_Trackback
class file and initializing an object instance of the Services_Trackback class.
Next, the set() method of the class sets the various
attributes of the trackback from an associative array: the trackback URL, the
post title, post URL, blog name, and an excerpt from the post to quote in
the trackback. Finally, the send() method of the
object actually creates a POST packet containing the trackback request and
transmit it to the trackback URL. If the return value from send() is not a PEAR Error, a success notification displays; if it
is, the user sees an error notification containing the text of the XML error
message.
A Simple Application
It's quite easy to use the Services_Trackback package in an interactive manner.
You create a Web form that allows users to post trackbacks to any blog that supports them. Listing 5 demonstrates:
Listing 5: A Web form to interactively set a trackback
<html>
<head></head>
<body>
<h2>Post a Trackback</h2>
<?php if (!isset($_POST['submit'])) { ?>
<form method="post" action="track3.php">
Trackback URL:
<br/>
<input type="text" name="tb_url" size="30" />
<p/>
Blog name:
<br/>
<input type="text" name="my_blog_name"/>
<p/>
Post title:
<br/>
<input type="text" name="my_post_title"/>
<p/>
Post excerpt:
<br/>
<textarea name="my_post_excerpt" cols="30" rows="5"></textarea>
<p/>
Post URL:
<br/>
<input type="text" name="my_post_url" size="30" />
<p/>
<input type="submit" name="submit" />
</form>
<?php
} else {
// encode POST data
$title = $_POST['my_post_title'];
$excerpt = $_POST['my_post_excerpt'];
$url = $_POST['my_post_url'];
$tburl = $_POST['tb_url'];
$name = $_POST['my_blog_name'];
$trackbackData = array(
'title' => $title,
'excerpt' => $excerpt,
'url' => $url,
'blog_name' => $name,
'trackback_url' => $tburl
);
// include class
include "Services/Trackback.php";
// initialize new instance
$trackback = new Services_Trackback();
// set object properties
foreach ($trackbackData as $k => $v) {
$trackback->set($k, $v);
}
// send trackback
$ret = $trackback->send();
if (PEAR::isError($ret)) {
echo "ERROR. Trackback failed because: " . $ret->getMessage();
} else {
echo "Trackback successfully posted";
}
}
?>
</body>
</html>
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Listing 5 generates a Web form that asks the user to input various attributes for a trackback: the trackback URL, post title and URL, blog name, and excerpt. This information is then submitted to a PHP script that initializes a new Services_Trackback object, and posts the trackback to the specified URL. The XML response is then decoded and a success or failure notification displayed.
Figure 1 illustrates what the Web form looks like:
Figure 1. A Web form to interactively generate a trackback
As this example illustrates, the Services_Trackback package provides an easy way to manually create trackbacks to blog post requests. It's very useful when you need to generate a trackback to your content, but don't necessarily have a blog to do this with. Play with it some and see what you think!
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