it was not clear from your questions that you were using the post method in the form or i missed it. the same code with method="post":

index.php

Code:
<?php
header("Cache-Control: no-store");
header("Expires: " . date("r"));
header("Cache-Control: max-age=0");
header("Cache-Control: no-cache");
header("Expires: 0");
header("Expires: Tue, 01 Jan 1970 1:00:00 GMT");
header("Pragma: no-cache");
?>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<title> Untitled </title>
<style type="text/css">
select {display:none;margin-top:5px;margin-bottom:5px;}
</style>
<script>
var args=['1,2,5,6','2,3,4,5,6','1,2,6,12','1,2,3,4,5,6,11'];
function $(id){return document.getElementById(id);}

window.onload=function(){
var i=0,
    loc=location.search.substr(1);
while($('r'+i)!==null){
$('r'+i).onclick=function(){
var index=$('predef').value=this.id.substr(1),
    arr=args[index].split(','),
    sel=$('selectBoxes').getElementsByTagName('select');
for(var k=0; k<sel.length; k++){sel[k].style.display='none';}
for(var s=0; s<arr.length; s++){
var sbox=Number(arr[s])-1;
$('SB'+sbox).style.display='block';
}
}
i++;
}

$('frm').onreset=function(){$('predef').value='';}
//var preval=loc.split('&')[0].split('=')[1];
//if($('r'+preval)!==null){$('r'+preval).click();}
<?php
if(isset($_POST['predef'])){
echo '$("r'.$_POST['predef'].'").click();
';
}
?>
}
</script>
</head>
<body>
<form name="frm" id="frm" action="" method="post">
<input type="hidden" name="predef" id="predef" />
<input type="radio" name="Rbtn" id="r0" /> Items 1,2,5,6 <br />
<input type="radio" name="Rbtn" id="r1" /> Items 2,3,4,5,6 <br />
<input type="radio" name="Rbtn" id="r2" /> Items 1,2,6,12 <br />
<input type="radio" name="Rbtn" id="r3" /> Items 1,2,3,4,5,6,11 <br />
<p>
<div id="selectBoxes">
<select id="SB0">  <option>SB00</option> <option>SB01</option> <option>SB02</option> </select>
<select id="SB1">  <option>SB10</option> <option>SB11</option> <option>SB12</option> </select>
<select id="SB2">  <option>SB20</option> <option>SB21</option> <option>SB22</option> </select>
<select id="SB3">  <option>SB30</option> <option>SB31</option> <option>SB32</option> </select>
<select id="SB4">  <option>SB40</option> <option>SB41</option> <option>SB42</option> </select>
<select id="SB5">  <option>SB50</option> <option>SB51</option> <option>SB52</option> </select>
<select id="SB6">  <option>SB60</option> <option>SB61</option> <option>SB62</option> </select>
<select id="SB7">  <option>SB70</option> <option>SB71</option> <option>SB72</option> </select>
<select id="SB8">  <option>SB80</option> <option>SB81</option> <option>SB82</option> </select>
<select id="SB9">  <option>SB90</option> <option>SB91</option> <option>SB92</option> </select>
<select id="SB10"> <option>SBA0</option> <option>SBA1</option> <option>SBA2</option> </select>
<select id="SB11"> <option>SBB0</option> <option>SBB1</option> <option>SBB2</option> </select>
</div>
<div><input type="submit" value="Submit" />&nbsp;&nbsp;&nbsp;<input type="reset" value="Reset" /></div>
</form>
</body>
</html>