And a new one that isn't public yet. 

 Parameter       Value                                Description                          
 appid     string (required) The application ID. See Application IDs for more information.
 context   string (required) The context to extract terms from (UTF-8 encoded).
 query     string            An optional query to help with the extraction process.
Example
<?php
$context = "Italian sculptors and painters of the renaissance favored the Virgin Mary for inspiration.";
$url = 'http://api.search.yahoo.com/ContentAnalysisService/V1/termExtraction';
$post = "query=madonna&appid=RESTDemo&context=".rawurlencode($context);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
$xml = simplexml_load_string(curl_exec($ch));
curl_close($ch);
echo $xml->asXML();
?>
Output