how to retrieve value of a json check box using php

I have a json form field which has a check box as coded :

{"name":"Act","description":"Checkthis","type":"checkbox"}
 

can anyone tell me how to get this value of this check box IF it is checked using php

here is the form:

<iframe src='http://www.facebook.com/plugins/registration.php?
                     client_id=360&
                     redirect_uri=http://www.pingcampus.com/facebook_registration_plugin/store_user_data.php&
                     fields=[
                     {"name":"name"},
                     {"name":"email"},
                     {"name":"gender"},
                     {"name":"birthday"},
                     {"name":"captcha"},
                     {"name":"Act","description":"Checkthis","type":"checkbox"}
                     ]'
                     scrolling="auto"
                     frameborder="no"
                     style="border:none"
                     allowTransparency="true"
                     width="500"
                     height="600">
             </iframe>
 

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