Populate one dropdown list based on the selection of other dropdown list and save with php

Question

In this link, how to save types of car and types of phones in database with php?

I mean how to save Mercedes, Volvo, BMW, porche from car and Samsung, Nokia, Iphone from phone to database?

Below is my code :
html:

 <select id="cat">
        <option val="car">car</option>
        <option val="phone">phone</option>
    </select>

    <select id="item">

    </select>

script:

 cars=new Array("Mercedes","Volvo","BMW","porche");
phones=new Array('Samsung','Nokia','Iphone');

populateSelect();

$(function() {

      $('#cat').change(function(){
        populateSelect();
    });

});


function populateSelect(){
    cat=$('#cat').val();
    $('#item').html('');


    if(cat=='car'){
        cars.forEach(function(t) { 
            $('#item').append('<option>'+t+'</option>');
        });
    }

    if(cat=='phone'){
        phones.forEach(function(t) {
            $('#item').append('<option>'+t+'</option>');
        });
    }

}