moo's JSON.encode and php's json_decode

[wanlee]

I'm trying the following code and my php returns null. What am I doing
wrong?

my JS:
var valsJSON = JSON.encode({min: '50', max: '200', betMin: '60',
betMax: '100'});

var getPayoff = new Request({
     url: 'bet.php?json=',
     method: 'get',
     onSuccess: function(response){
          //$('payoff').set('html', response);
          alert(response);
     },
     onFailure: function(){
          $('payoff').set('html', 'fail');
     }
});
getPayoff.send(valsJSON);


my PHP:
<?php
$json_data = $_GET["json"];

// another post said the following string_repace might solve but
instead it causes the entire script to fail
//string_replace('\"', '"', $json_data);
var_dump(json_decode($json_data));

?>