如何在第二个下拉列表中获取多个选定的值

i am new in php coding.i am facing a problem.i have a drop down list where user can select multiple values.Like i have a drop down list of different writers.suppose user select two or more writers from the first drop down list.In 2nd drop down list should show all the books of the both the writer.All the values(writers and books) are coming from database.Thanks all in advance.

This snippet will give you multiple value from ONE dropdown:

<form method="post">
    <select id="mySelectID" name="mySelectID[]" multiple>
        <option value="volvo">Volvo</option>
        <option value="saab">Saab</option>
        <option value="opel">Opel</option>
        <option value="audi">Audi</option>
    </select><br />
    <input type="submit" value="Send" />
</form>
<pre>
<?php
var_export($_POST);
?>
</pre>

After a POST, you will get:

array (
  'mySelectID' => 
  array (
    0 => 'saab',
    1 => 'opel',
  ),
)

If you want to interact with other dropdown menu without reload, you need to lookat JavaScript.

UPDATE

A basic sample with dropdown interaction without reload http://jsfiddle.net/hu8gf4gb/1/