Non-static method YahooSession::requireSession() should not be called statically

I am using yahoo / yos-social-php for login to my site i tried simple code but it resulted following error

Strict Standards: Non-static method YahooSession::requireSession() should not be called statically in /nas/www/inwiter/lib/yahoo/login.php on line 13

**My code is **

<?php  
  require("lib/Yahoo.inc");  

  // Your Consumer Key (API Key) goes here.  
  define('CONSUMER_KEY', "********");  

  // Your Consumer Secret goes here.  
  define('CONSUMER_SECRET', "*****");  

  // Your application ID goes here.  
  define('APPID', "*****");  

  $session = YahooSession::requireSession(CONSUMER_KEY,CONSUMER_SECRET,APPID);
  print_r($session);  
?>  

Can any one suggest me where i am wrong.