I do a lot of work with the Smarty template engine, and the site I'm working on now is large, complex, and has a fairly advanced layout with lots of CSS and jQuery goodies. Everything is so abstracted, it's just not practical to construct a single template for my Authorize.net relay pages, of which there are several. It's also not practical to change the templates to use fully qualified URLs for everything. So what I did was write a simple Smarty modifier that prepends a specified URL to all of the relative src, href, and action attributes within the supplied string. It also fixes background: url() inline CSS styles. On my relay templates, all I have to do is wrap the contents of the template in a {capture} function, assign the output to a variable, and run it through my modifier. Absolute paths for all. Here's the modifier code, hopefully it will save someone a few grey hairs.

<?php
/**
 * Smarty plugin
 * @package Smarty
 * @subpackage plugins
 */

/**
 * Smarty base_href modifier plugin
 *
 * Type:     modifier<br>
 * Name:     base_href<br>
 * Purpose:  apply a base URL to all relative
 *				src, href, and background:url() attributes in a string
 * @author   Rob Ruchte <rob care of thirdpartylabs com>
 * @param string
 * @param string
 * @return string
 */
function smarty_modifier_base_href($string, $base_href)
{
	$base_href = (substr($base_href, -1)!='/') ? $base_href.'/':$base_href;

	$patterns = array();
	$patterns[] = '/href=\"[^#]\/?([^\"]*)\"/i';
	$patterns[] = '/src=\"\/?([^\"]*)\"/i';
	$patterns[] = '/action=\"\/?([^\"]*)\"/i';
	$patterns[] = '/background: ?url\(\'?\/?([^\'|\)]*)\'?\)/i';
	$patterns[] = '/background-image: ?url\(\'?\/?([^\'|\)]*)\'?\)/i';

	$replacements = array();
	$replacements[] = 'href="'.$base_href.'$1"';
	$replacements[] = 'src="'.$base_href.'$1"';
	$replacements[] = 'action="'.$base_href.'$1"';
	$replacements[] = 'background: url(\''.$base_href.'$1\')';
	$replacements[] = 'background-image: url(\''.$base_href.'$1\')';

    return preg_replace($patterns, $replacements, $string);
}

/* vim: set expandtab: */

?>

Usage:

{capture assign=output}
{include file="include/header.tpl" title="Event Ticket Registration"}
	<div class="span-3" id="leftbar">
		[...]
	</div>
	<div class="span-11 last" id="content">
		<div style="background: url(/images/feature.jpg);">
			[...]
		</div>
		<div class="span-11 last" id="copy">
		<div id="breadcrumbs">
			{foreach from=$breadcrumbs item=currNav name=breadcrumbs}
				{if !$smarty.foreach.breadcrumbs.last}<a href="{$currNav.path}">{/if}
				{$currNav.title}
				{if !$smarty.foreach.breadcrumbs.last}</a>&nbsp;&raquo;&nbsp;{/if}
			{/foreach}
		</div>
        <h2>Registration: {$event->getTitle()|escape}</h2>
	    {$purchaseSuccess}
        </div>
    </div>
{include file="include/footer.tpl"}
{/capture}
{$output|base_href:$smarty.const.BASE_URL}

In our case, we are proxying the Atom feed through a PHP script so we can display the feed contents to user agents without the Flash player. This made it fairly easy to iterate through the entries, and with a simple bit of regex, strip out the offending markup from the contents.

Blogger is wrapping the image tag in a div with a very specific CSS class, which makes our job easy:

Depending on what you're using to parse the feed, you may or may not need to be concerned about decoding and encoding html entities during this process.

Usage:
{$fileSizeInBytes|file_size}

Example:
{assign var=fileSizeInBytes value=10485760}
{$fileSizeInBytes|file_size}

{assign var=fileSizeInBytes value= 768000}
{$fileSizeInBytes|file_size}

{assign var=fileSizeInBytes value=303}
{$fileSizeInBytes|file_size}

Output:
10 MB
750 Kb
303 bytes

<?php
/**
 * Smarty plugin
 * @package Smarty
 * @subpackage plugins
 */

/**
 * Smarty file_size modifier plugin
 *
 * Type:     modifier<br>
 * Name:     file_size<br>
 * Purpose:  format file size represented in bytes into a human readable string<br>
 * Input:<br>
 *         - bytes: input bytes integer
 * @author   Rob Ruchte <rob at thirdpartylabs dot com>

 * @param integer
 * @return string
 */
function smarty_modifier_file_size($bytes=0)
{
    $mb = 1024*1024;

    if ($bytes > $mb)
    {
        $output = sprintf ("%01.2f",$bytes/$mb) . " MB";
    }
    elseif ( $bytes >= 1024 )
    {
        $output = sprintf ("%01.0f",$bytes/1024) . " Kb";
    }
    else
    {
        $output = $bytes . " bytes";
    }

    return $output;
}

/* vim: set expandtab: */

?>

Getting PEAR, PECL, and a build environment set up can be a time consuming process if you do it from scratch. Luckily most OSs these days come with easy to use package management systems that take care of downloading, building, and installing the packages you need, and their dependancies.

Check the package management system entry on Wikipedia if you're unsure about which package system your OS uses. Each one works a little bit differently, but the concepts are the same. You're going to need to set up your OS's version of the following packages in order to make your life easy:

php-devel
php-pear
mysql-devel
httpd-devel

The package names will differ a bit for each system. The following example uses the Yum package manager in CentOS, and gets PDO_MYSQL up and running in just a few minutes.

# yum install php-devel php-pear mysql-devel httpd-devel
# pecl install pdo
# PHP_PDO_SHARED=1 pecl install pdo_mysql

Add these lines to php.ini:

extension=pdo.so
extension=pdo_mysql.so

Now restart Apache, and you should see your PDO modules in phpinfo(), Robert's your father's brother.
# apachectl restart

If you need to actually make a custom PHP build, you should have everything you need to do that after installing the devel packages. You still may need to build any dependancies that your custom PHP build requires, but your package system should make that fairly easy.