For a backup utility I needed link-like functionality on a windows system. As it isn't availible on windows, i tried to do it myself with the help of some tools. All you need is junction.exe from sysinternals in your %PATH%.

<?php
if(!function_exists('link')){ // Assume a windows system
    function link($target, $link){
        if(is_dir($target)){
            // junctions link to directories in windows
            exec("junction $link $target", $lines, $val);
            return 0 == $val;
        }elseif(is_file($target)){
            // Hardlinks link to files in windows
            exec("fsutil hardlink create $link $target", $lines, $val);
            return 0 == $val;
        }
        
        return false;
    }
}
?>

http://www.sysinternals.com/Utilities/Junction.html

to clarify:

in unix/linux:
hardlinks (by this function) cannot go across different filesystems.
softlinks can point anywhere.

in linux, hardlinking to directory is not permited.

I noticed that, differently from Unix ln command, the second parameter can´t be a directory name, i.e., if you want to create a link with the same filename of the target file (obviously on different directories), you must specify the filename on the link parameter.

Example:
Unix ln command:
ln /dir1/file /dir2/   // ok, creates /dir2/file link

PHP link function:
link ("/dir1/file", "/dir2/");   // wrong, gives a "File exists" warning
link ("/dir1/file", "/dir2/file");   // ok, creates /dir2/file link