Generate an absolute link from a URL plus a link

It sounds like you want something like realpath()
http://us2.php.net/realpath

something like (assuming your variable is in $var)

<?php
//change these to match your server
$base_path = '/absolute/path/to/www/dir';
$base_url = 'http://www.yoursite.com';

$var_real = str_replace($base_url, $base_path, $var);
$var_real = realpath($var_real);
$var_real = str_replace($base_path, $base_url, $var_real);

echo $var_real;
?>

This is untested, but should be the right idea.

It probably also makes sense to check if someone has put in so man ../../.. that it makes it traverse back below your web directory, and to make sure realpath is not false...putting that all together in a function,

function get_real_path($var){
  //change these to match your server  
  $base_path = '/absolute/path/to/www/dir';
  $base_url = 'http://www.yoursite.com';

  $var_real = str_replace($base_url, $base_path, $var);
  $var_real = realpath($var_real);

  if ($var_real === false || strpos($var_real, $base_path) !== 0) return false;

  return  str_replace($base_path, $base_url, $var_real);
}

<?php
function get_absolute($baseUrl, $relativeLink)
{
    // Getting base scheme, host and path. If base url is not correct
    // return false
    if (!($base = @parse_url(trim($baseUrl)))) return false;
   
    // Checking relativeLink
    if (1 === preg_match("/\/[^.]+\/\.+/", $relativeLink)) return false;
   
    // Parsing relativeLink
    $rel    = explode("/", trim($relativeLink));
   
    // Now we have to know how deep is relative
    $level_up   = 0;
    $level_root = false;
    foreach ($rel as $k => $v)
    {
        if ("" == $v)
        {
            unset($rel[$k]);
        }
        if ("." == $v)
        {
            $level_root = true;
            unset($rel[$k]);
        }
        if (".." == $v)
        {
            $level_up++;
            unset($rel[$k]);
        }
    }
   
    $path = "";
    if ($level_root)
    {
        $path = "/" . implode("/", $rel);
    }
    else
    {
        $base['path'] = preg_replace("/^\/+/", "", $base['path']);
        $base['path'] = preg_replace("/\/+$/", "", $base['path']);
        $base['path'] = explode("/", $base['path']);
        if ($level_up >= count($base['path']))
        {
            $path = "/" . implode("/", $rel);
        }
        else
        {
            while($level_up != 0)
            {
                $i = count($base['path']) - 1;
                unset($base['path'][$i]);
                $level_up--;
            }
            $path = "/" . implode("/", $base['path']) . "/" . implode("/", $rel);
        }
    }
   
    return $base['scheme'] . "://" . $base['host'] . $path;
}

echo get_absolute('http://foo.com/bar/', '../pics/my.gif');
?>

Thanks guys. I'm splitting the points. The first led to me generating a solution(before ixti posted) and the second is also a useful solution which I'm going to look at more carefully.