Output javascript value in php based on radio buttons

I would like to output javascript value in a php code. It seems easy but the javascript variables must be based on the answers of a form of the document.

For exemple : If first radio button is checked, outputs first value in php code If second radio button is checked, output second value in php code.

<div class="test">
           <div>
            <input type="radio" id="troismois"
                   name="testradio" value="troismois" checked />
            <label for="troismois">3 mois</label>
        </div>

        <div>
            <input type="radio" id="unmois"
                   name="testradio" value="unmois" />
            <label for="unmois">1 mois</label>
        </div>
<script> 
    var checkBox = document.getElementById("huey");

     if (checkBox.checked == true){
   var x = "10";
  } else {
    var x = "20";
  } 
</script>
<?php 
$content = "<script> document.write(x) </script>";

echo $content;
?>

But it's working, I miss something but I don't know what, maybe someone can help me?

Thank you !