Perhaps this article could help?
http://nashruddin.com/PHP_Script_for_Converting_Relative_to_Absolute_URL
Edit: reproduced code below for convenience
<?php
function rel2abs($rel, $base)
{
/* return if already absolute URL */
if (parse_url($rel, PHP_URL_SCHEME) != '') return $rel;
/* queries and anchors */
if ($rel[0]=='#' || $rel[0]=='?') return $base.$rel;
/* parse base URL and convert to local variables:
$scheme, $host, $path */
extract(parse_url($base));
/* remove non-directory element from path */
$path = preg_replace('#/[^/]*$#', '', $path);
/* destroy path if relative url points to root */
if ($rel[0] == '/') $path = '';
/* dirty absolute URL */
$abs = "$host$path/$rel";
/* replace '//' or '/./' or '/foo/../' with '/' */
$re = array('#(/\.?/)#', '#/(?!\.\.)[^/]+/\.\./#');
for($n=1; $n>0; $abs=preg_replace($re, '/', $abs, -1, $n)) {}
/* absolute URL is ready! */
return $scheme.'://'.$abs;
}
?>