new google recaptcha with checkbox server side php

i just set up the new google recaptcha with checkbox, it's working fine on client side, however i don t know how to do it in server side using php, the form is submetted even if the checkbox isn't checked... i have two files contact.html (html form) and contact.php server side file. please help me it's urgent.... thank you. -- You received this message because you are subscribed to the Google Groups "reCAPTCHA" group. To unsubscribe from this group and stop receiving emails from it, send an email to recaptcha+unsubscribe@googlegroups.com. To post to this group, send email to recaptcha@googlegroups.com. Visit this group at http://bit.ly/1dkFnYd. For more options, visit http://bit.ly/P65DvS.