我这样做是这样的:

$signed_request = $_REQUEST['signed_request'];  

function parsePageSignedRequest() {  
    if (isset($_REQUEST['signed_request'])) {  
      $encoded_sig = null;  
      $payload = null;  
      list($encoded_sig, $payload) = explode('.', $_REQUEST['signed_request'], 2);  
      $sig = base64_decode(strtr($encoded_sig, '-_', '+/'));  
      $data = json_decode(base64_decode(strtr($payload, '-_', '+/'), true));  
      return $data;  
    }  
    return false;  
}  


if($signed_request = parsePageSignedRequest()) {  
    if($signed_request->page->liked) {  
     echo "FAN";  
    } else {  
      echo "NO FAN";  
    }  
}