(PECL eio >= 0.0.1dev)
eio_link — Create a hardlink for file
$path
   , string $new_path
   [, int $pri = EIO_PRI_DEFAULT
   [, callable $callback = NULL
   [, mixed $data = NULL
  ]]] )
  eio_link() creates a hardlink
  new_path for a file specified by
  path.
  
pathSource file path.
new_pathTarget file path.
priThe request priority: EIO_PRI_DEFAULT, EIO_PRI_MIN, EIO_PRI_MAX, or NULL.
If NULL passed, pri internally is set to
EIO_PRI_DEFAULT.
callback
callback function is called when the request is done.
It should match the following prototype: 
void callback(mixed $data, int $result[, resource $req]);
datais custom data passed to the request.
resultrequest-specific result value; basically, the value returned by corresponding system call.
reqis optional request resource which can be used with functions like eio_get_last_error()
data
     Arbitrary variable passed to callback.
     
Example #1 eio_link() example
<?php
$filename = dirname(__FILE__)."/symlink.dat";
touch($filename);
$link = dirname(__FILE__)."/symlink.link";
$hardlink = dirname(__FILE__)."/hardlink.link";
function my_hardlink_cb($data, $result) {
    global $link, $filename;
    var_dump(file_exists($data) && !is_link($data));
    @unlink($data);
    eio_symlink($filename, $link, EIO_PRI_DEFAULT, "my_symlink_cb", $link);
}
function my_symlink_cb($data, $result) {
    global $link, $filename;
    var_dump(file_exists($data) && is_link($data));
    if (!eio_readlink($data, EIO_PRI_DEFAULT, "my_readlink_cb", NULL)) {
        @unlink($link);
        @unlink($filename);
    }
}
function my_readlink_cb($data, $result) {
    global $filename, $link;
    var_dump($result);
    @unlink($link);
    @unlink($filename);
}
eio_link($filename, $hardlink, EIO_PRI_DEFAULT, "my_hardlink_cb", $hardlink);
eio_event_loop();
?>
The above example will output something similar to:
bool(true) bool(true) string(%d) "%ssymlink.dat"