javascript - Dropdown automatically selected a value -

i need problem. want make dynamically dropdown , when select value 1 dropdown "a", dropdown set "b".

i have javascript function dynamically dropdown this.

<script type="text/javascript"> function coba(){         document.getelementbyid("add").innerhtml +=   " <inputclass='department_name' type='text'     size='50' />";   }  </script> 

reference: how dynamically change item of 2 related combobox

in short:

1. in file1.php, retrieve mysql tbl1 , display in combo box a.
2. on change of combo box a, fetch value of option , pass php file file2.php via ajax , display output in file1.php produced file2.php.
3. in file2.php, retrieve mysql tbl2 id passed ajax , generate combo box b.

example:

index.php

<script type="text/javascript"> function getxmlhttpobject() {     if (window.xmlhttprequest)     {         return new xmlhttprequest();     }     if (window.activexobject)     {         return new activexobject("microsoft.xmlhttp");     }        return null; }  function ajax_function(url, postdata, id) {     xmlhttp=getxmlhttpobject();     xmlhttp.open("post", url, true);     xmlhttp.setrequestheader("content-type", "application/x-www-form-urlencoded");     xmlhttp.setrequestheader("content-length", postdata.length);     xmlhttp.setrequestheader("connection", "close");     xmlhttp.onreadystatechange=function()     {         if(xmlhttp.readystate==4)         {             document.getelementbyid(id).innerhtml=xmlhttp.responsetext;                                     }            }                         xmlhttp.send(postdata); }  function dispsecond(id) {     var params  = 'id=' + id ;     var divid = 'dispdiv';     ajax_function('ajax_display.php', params, divid); }  </script>  <?php /* mysqli query retrieve , store in $arraylist(id=>text)    example:  $arraylist = array(1=>'ford',2=>'chevy'); */ ?>  <select id="drop_first" name="drop_first" onchange="return dispsecond(this.value);"> <option value="0">[select]</option> <?php foreach ($arraylist $k=>$v) { echo '<option value="'.$k.'">'.$v.'</option>';   } ?> </select>  <div id="dispdiv"></div> 

ajax_display.php

<?php $id     = isset($_request['id']) ? $_request['id'] : ''; if ($id) { /* mysqli query retrieve , store in $subarray $id    example:      if $id=1     $subarray = array(1=>'focus',2=>'explorer');    if $id=2    $subarray = array(1=>'cavalier',2=>'impala', 3=>'malibu'); */ ?>     <select id="drop_second" name="drop_second">     <option value="0">[select]</option>     <?php     foreach ($subarray $k=>$v)     {     echo '<option value="'.$k.'">'.$v.'</option>';       }     ?>     </select> <?php } ?> 

note:

use mysqli or pdo instead mysql

below demo , download based on arrays, can implement using mysqli retrieval.

also can try using $.ajax more easy also.

demo | download