title: Multiple selection from two respective dropdown
body: <p>I have two dropdown in html page.1st dropdown Contain Class-1, class-2,class3..
  and 2nd dropdown contain StudentNameID for selected Classes. I wanted to multiple selection for classes in first dropdown and respective classes manage second dropdown selection for studentID.</p>
  
  <pre><code>1st Dropdown-want mutiple selection
  &lt;select&gt;
      &lt;option value=".$row['classID']."&gt;Class-1&lt;/option&gt;
      ....like wise generate dropdown...
      &lt;option&gt;Class-2&lt;/option&gt;
      &lt;option&gt;Class-3&lt;/option&gt;
      &lt;option&gt;Class-4&lt;/option&gt;
  &lt;/select&gt;  
  2nd Dropdown-On selection of 1st dropdown show 2nd dropdown value.
  &lt;select&gt; //same for 2nd dropdown list..
      &lt;option&gt;StudentNameID1-Class-1&lt;/option&gt;
      &lt;option&gt;StudentNameID2-Class-1&lt;/option&gt;
      &lt;option&gt;StudentNameID1-Class-2&lt;/option&gt;
      &lt;option&gt;StudentNameID2-Class-2&lt;/option&gt;
      &lt;option&gt;StudentNameID3-Class-2&lt;/option&gt;
      &lt;option&gt;StudentNameID1-Class-3&lt;/option&gt;
      &lt;option&gt;StudentNameID2-Class-3&lt;/option&gt;
      &lt;option&gt;StudentNameID3-Class-3&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>I want multiple selection is there and selected Id store in variable.So,by exploding I will use it on my next page where page redirect.</p>
  
  <p>My question Is I want multiple selection fron both dropdowns.
  If I choose Class-1 and Class-2 from 1st drop down then atomatic 2nd dropdown will shows related values from selected ClassID's.
  Also same multiple selection for 2nd dropdown.
  Can you please tell me How I will approched using php and java script?</p>