title: Combobox items based on selected item in another combobox
body: <p>If I have a <code>combobox1</code> has the items: Fruits &amp; vegetables.</p>
  
  <p>I need to show a nother <code>combobox2</code> based on the selected item in <code>combobox1</code>.</p>
  
  <p>If the selected item in <code>combobox1</code> is <code>Fruits</code>, then  <code>combobox2</code> items are: apple, orange ..etc.</p>
  
  <p>If the selected item in <code>combobox1</code> is <code>vegetables</code>, then  <code>combobox2</code> items are: radish, lettuce..etc.</p>
  
  <p>How can I do that using PHP &amp; HTML? (<strong><em>Please consider "PHP &amp; HTML only" as a condition</em></strong>).</p>