2-Legged using OAuth.inc from the PHP SDK
<?php
require './OAuth.php';
include 
'./oauth_secret.inc';

function 
get_2legged($url) {
  global 
$consumerKey$consumerSecret;

  list(
$base,$query) = explode('?',$url,2);
  
parse_str($query$args);

  
$consumer = new OAuthConsumer($consumerKey$consumerSecret);
  
$request OAuthRequest::from_consumer_and_token($consumer,NULL,"GET",$base,$args);
  
$request->sign_request(new OAuthSignatureMethod_HMAC_SHA1(), $consumerNULL); 

  
$ch curl_init();
  
curl_setopt($chCURLOPT_URL$url);
  
curl_setopt($chCURLOPT_HTTPHEADER, array($request->to_header()));
  
curl_setopt($chCURLOPT_RETURNTRANSFER1);
  return 
curl_exec($ch);
}

echo 
get_2legged('http://query.yahooapis.com/v1/yql?q=show%20tables&format=xml');
?>
Output
<?xml version="1.0" encoding="UTF-8"?>
<query xmlns:yahoo="http://www.yahooapis.com/v1/base.rng"
    yahoo:count="33" yahoo:created="2008-09-12T02:56:55Z"
    yahoo:lang="en-US" yahoo:updated="2008-09-12T02:56:55Z" yahoo:uri="http://query.pipes.yahoo.com/v1/yql?q=show+tables">
    <results>
        <tables>
            <table>atom</table>
            <table>csv</table>
            <table>flickr.location</table>
            <table>flickr.photos.exif</table>

            <table>flickr.photos.info</table>
            <table>flickr.photos.interestingness</table>
            <table>flickr.photos.recent</table>
            <table>flickr.photos.search</table>
            <table>flickr.places</table>
            <table>geo.places</table>

            <table>geo.placetypes</table>
            <table>html</table>
            <table>json</table>
            <table>local.search</table>
            <table>rss</table>
            <table>search.images</table>

            <table>search.news</table>
            <table>search.web</table>
            <table>social.connections</table>
            <table>social.contacts</table>
            <table>social.presence</table>
            <table>social.profile</table>

            <table>social.updates</table>
            <table>upcoming.category</table>
            <table>upcoming.country</table>
            <table>upcoming.events</table>
            <table>upcoming.events.bestinplace</table>
            <table>upcoming.groups</table>

            <table>upcoming.metro</table>
            <table>upcoming.state</table>
            <table>upcoming.venue</table>
            <table>weather.forecast</table>
            <table>xml</table>
        </tables>

    </results>
</query>
<!-- total: 1 -->
<!-- yqlengine2.pipes.sp1.yahoo.com uncompressed/chunked Fri Sep 12 07:56:55 PDT 2008 -->