differentiating identical select names - php

Question

how can I differentiate the 'select name' of 'f1' and 'f2' currently both named 'subcat' while still having only 1 subcat variable? this code works accurately only if cat value=2, if cat value=1 then subcat value always =0
<?php
$cat=$_POST['cat'];
$subcat = $_POST['subcat'];
?>
<form action='submitsite.php' method='POST'>
<table>
<tr>
<td>category(optional)</td>
<td>
<select name='cat' id = "opts" onchange = "showForm()">
<option value = "0">Select</option>
<option value = "1">music </option>
<option value = "2">film </option>
</select>
<div id = "f1" style="display:none">
<select name='subcat' id = "opts" onchange = "showForm()">
<option value = "0">Select</option>
<option value = "3">pop</option>
<option value = "4">rock </option>
</select>
</div>
<div id = "f2" style="display:none">
<select name='subcat' id = "opts" onchange = "showForm()">
<option value = "0">Select</option>
<option value = "5">comedy</option>
<option value = "6">drama</option>
</select>
</div>
</form>
</div>
<script type = "text/javascript">
function showForm(){
var selopt = document.getElementById("opts").value;
if (selopt == 1) {
document.getElementById("f1").style.display="block";
document.getElementById("f2").style.display="none";
}
if (selopt == 2) {
document.getElementById("f2").style.display="block";
document.getElementById("f1").style.display="none";
}
}
</script>

Answer 1

Add [] to the name. The selects will then be interpreted as an array when submitted.
<select name="subcat[]">...</select>
With PHP it can be accessed (if POSTed) like this:
<?php
$subCatArr = $_POST['subcat'];
$firstIndex = $subCatArr[0];
$secondIndex = $subCatArr[1];
Oh yeah, and reusing IDs in HTML is not valid. They must be unique.
UPDATE After better understanding OP's intent:
If I understand the intent of this spaghetti code (that's a term of endearment, OP), the user may select one category and one subcategory, which is based on the category that he/she selected. Then the user submits the form and that selection is recorded in the database.
First of all, let's get rid of the use of table elements, because they're unnecessary. Secondly, you only need one select for the subcategory.
<form action='submitsite.php' method='POST'>
<label>category(optional)</label>
<select name='cat' id = "opts" onchange = "showForm()">
<option value = "0">Select</option>
<option value = "1">music </option>
<option value = "2">film </option>
</select>
<div id="subcatDiv" style="display:none;">
<select name='subcat' id='opts'></select>
</div>
.
.
.
<input type='submit' />
</form>
We can leave it blank, since it's not being displayed anyway.
Now, when the user makes a change, we'll either display the appropriate subcategories, or we'll just hide the div again:
<script type = "text/javascript">
function showForm(){
var selopt = document.getElementById("opts");
var seloptVal = selOpt.value;
var subcatDiv = document.getElementById("subcatDiv");
var options = "";
switch(seloptVal * 1) {
case 1:
options = "<option value='0'>Select</option>" +
"<option value='3'>pop</option>" +
"<option value='4'>rock</option>";
break;
case 2:
options = "<option value='0'>Select</option>" +
"<option value='5'>comedy</option>" +
"<option value='6'>drama</option>";
break;
default:
break;
}
if (options == "") {
subcatDiv.style.display = "none";
selopt.innerHTML = options;
} else {
selopt.innerHTML = options;
subcatDiv.style.display = "block";
}
}
</script>
Now you only have to deal with one select for the subcategory.

Answer 2

I have altered a lot of your code to make it valid html and also work as i "think" you want it as your question did not make it very clear.
Here is a list of amends I had to make:
Changed ID's to make it clear what they are for.
Removed duplicate ID's.
Closed off your table properly.
Completely changed your JavaScript to show and hide certain select's
Many more things that I have forgotten.
Please see this jsfiddle.
Fixed html:
<form action='submitsite.php' method='POST'>
<table>
<tr>
<td>category(optional)</td>
<td>
<select name="cat" id="selectType">
<option value="0">Select</option>
<option value="1">music</option>
<option value="2">film</option>
</select>
<div id="f1" style="display:none">
<select name='music' id="selectMusic">
<option value="0">Select</option>
<option value="3">pop</option>
<option value="4">rock</option>
</select>
</div>
<div id="f2" style="display:none">
<select name="type" id="selectFilm">
<option value="0">Select</option>
<option value="5">comedy</option>
<option value="6">drama</option>
</select>
</div>
</td>
</tr>
</table>
</form>
Fixed JavaScript:
$("#selectType").change(function() {
var selected = $(this).val();
$("#f1, #f2").hide();
switch (selected) {
case "1":
$("#f1").show();
break;
case "2":
$("#f2").show();
break;
}
});​​

Answer 3

document.forms[0].subcat will be an array.
document.forms[0].subcat[0] will be the first instance, etc.
p.s. IDs are supposed to be unique.

Answer 4

document.getElementById('f2').getElementsByTagName('select')[0]
You are violating HTML standards by reusing IDs, by the way.