Invalid verification code format

PHP
1

I m trying to access facebook access token. i am new at this stuff. i was able to part 1 of it i.e making my app asking permission but i m stuck on next page. when i click the 2nd page link it gives me error saying

{    "error": {       "message": "Invalid verification code format.",       "type": "OAuthException"    } }

here is my code of 2nd page on which after click i get the above mentioned error

<a href="https://graph.facebook.com/oauth/access_token?
     client_id=123&redirect_uri=http://www.abc.com/fbml.php/&
     client_secret=c12&code={$_REQUEST['code']}">Click</a>

am i doing something wrong?