As rythmic said, normally use javascript on the client side.
Heres some code to play with...
Code:
<html>
<head>
<script>
model_arr = new Array();
model_arr[0] = new Array('Focus', 'Escort', 'Mondeo', 'Transit');
model_arr[1] = new Array('Clio', 'Scenic', 'Megane', '5 Turbo');
model_arr[2] = new Array('Chevette', 'Vectra', 'Astra', 'Belmont');
function manuf_changed()
{
// Remove all previous options from models drop down
while(models.options.length > 0)
models.options.remove(0);
// Add a new option element for each element in the model sub-array
for(var i=0; i<model_arr[manufs.value].length; i++)
{
var oOption = document.createElement("OPTION");
models.options.add(oOption);
oOption.innerText = model_arr[manufs.value][i];
oOption.value = "";
}
}
</script>
</head>
<body>
<select name="manufs" onchange="manuf_changed()" >
<option value="0">Ford</option>
<option value="1">Renault</option>
<option value="2">Vauxhall</option>
</select>
<br>
<br>
<select name = "models">
<option value="0">Pick a model</option>
</select>
</body>
</html>