How to transmit values between different forms

How to transmit values between different forms? & Lt ;? Phpecho & quot; & nbsp; & lt; formname = f1mothod = GET & gt; & lt; inputtype = checkboxname = C1checked & gt; C1 & lt; br & gt; & lt; inputtype = checkboxname how to transmit values between different forms?
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$ A = $ _ GET [???];
Echo $;
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The code is as above. because of layout requirements, the check box and the submit button are in different forms. how can I write the code to click submit in f2 to get the selected f1 data? How did they transmit the information?

By the way, the above $ a =$ _ GET [???], I want to express $ a = to get data, but I won't write it. the format is correct. thank you for your advice.

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Use js to register the subibd event when you click f2 (other events are acceptable), and then process the above checkbox in the event, for example, use jquery $ ('input [type = checkbox] '). attr ('checked', 'checked ');
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Print_r ($ _ GET); // you will know the value you want to obtain.