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}

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: */

?>