So you want to generate dynamic links according to user input.
The easiest way is use java script.
html
<select id="first">
<option value="link1">link1</option>
<option value="link2">link2</option>
<option value="link3">link3</option>
<option value="link4">link4</option>
</select>
<select id="second">
<option value="sublink1">sublink1</option>
<option value="sublink2">sublink2</option>
<option value="sublink3">sublink3</option>
<option value="sublink4">sublink4</option>
</select>
<!--call javascript function-->
<button type="button" onclick="getUrl();">Go to link</button>
javascript
<script>
function getUrl(){
var first= document.getElementById('first');
var second = document.getElementById('second');
var first_val=first.options[first.selectedIndex];
var second_val=second.options[second.selectedIndex];
var url=first_val.value+"/"+second_val.value;
window.location.href=url;
}
</script>