Yahoo provides a term extraction web service by which you can extract important keyphrases from a given text.The service is accessible via Yahoo Boss.You will need a Yahoo! BOSS Application ID to use it. It’s free of cost, so get it 
Then you can use the PHP cURL function to post the variables to the specified URL and return the result to a variable($xml here). The function I used if pasted below:
PHP Function:
function getKeyphrases($texttoprocess){
// cURL must be installed for this to work
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, ‘http://api.search.yahoo.com/ContentAnalysisService/V1/termExtraction’); curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt( $ch, CURLOPT_POSTFIELDS, ‘appid=PASTE YOUR YAHOO BOSS APP ID HERE&context=’ . urlencode($texttoprocess) );
$xml = curl_exec($ch) or die(‘error’);
curl_close($ch);
return $xml
}
The sample working code with HTML GUI could be downloaded from here.(Don’t forget to add your App ID in yahoo.php file)
