Populate another select dropdown from database based on dropdown selection
===

I am building a website to learn coding and am trying to build a tool where a user clicks on a select/dropdown that contains some category names pulled from database **cat** and then another select will appear with subcategory names pulled from database **subcat**. This is almost exactly like Yelp's (go down to the categories) [like Yelp's (go down to the categories)][1].

I also made a diagram:

![enter image description here][2]

I already have a category dropdown that is pulling from **cat** database:

    <p><b>Category:</b><br />
     <?php
      $query="SELECT id,cat FROM cat";
      $result = mysql_query ($query);
      echo"<select name='cselect3' class='e1'><option value='0'>Please Select A       Category</option>";
      // printing the list box select command
      while($catinfo=mysql_fetch_array($result)){//Array or records stored in $nt
      echo "<option value=\"".htmlspecialchars($catinfo['cat'])."\">".$catinfo['cat']."    </option>";

      }

    echo"</select>";
    ?>

And I have a subcat that is pulling from subcat database:

    <p><b>Subcat1:</b><br />
    <?php
      $query="SELECT id,subcat FROM subcat";
      $result = mysql_query ($query);
   	  echo"<select name='sselect1' class='e1'><option value='0'>Please Select A Category</option>";
      // printing the list box select command
      while($catinfo=mysql_fetch_array($result)){//Array or records stored in $nt
          echo "<option value=\"".htmlspecialchars($catinfo['subcat'])."\">".$catinfo['subcat']."</option>";

      }

     echo"</select>";
    ?>

  [1]: http://www.yelp.com/biz_attribute?biz_id=Jsuy0DN6NhILXSkHr6UpCA
  [2]: http://i.stack.imgur.com/2dtlp.png

How do I make a subcategory dropdown based on what the user clicks on category and make it automatically appear?
Thanks so much for any and all help!