$url = 'about-us';
$path = drupal_lookup_path('source', $url);

if ($path) {
  $node = menu_get_object('node', 1, $path);
  return $node->nid;
}
else {
  //Error handling in case path provided does not exist
}

$argument_one = arg(0); // This will store "node" to the variable.
$node_id = arg(1); // This will store 100 to $node_id variable.

$URL = $_SERVER['REQUEST_URI'];
OR
$URL = $_GET['q']
// if using this then the full path needs prepending as this only gets node/nid
OR
$URL = current_path(); // same as GET[q] basically

// then this passes to that function you wanted
$nid = getNodeId($URL);

$nid = end(arg()); // Get the last number from the URL.

$url_arr = explode('/', $url);

<?php
    $nodeid = $node->nid;
    ?>

<a href="http://www.mysite.com?nodeid="<? php print ($nodeid) ?>">

<?php
$nodeid=$_GET["nodeid"];
?>