to pass the html combobox value to php in same file without submit -

i want change combobox2 entry based on selection in combobox1.. want retrieve entries of combobox2 database , need in php. need pass selected value in combobox1 php php variable. tried lot... not yet got output. here code

degree type<select id="select3" name="deg">             <option value="ug">ug</option>              <option value="pg">pg</option>         </select> department name<?php           $host="localhost" ;      $mysql_db="db" ;      $mysql_u="root" ;      $mysql_p="" ;      mysql_connect( "$host", "$mysql_u", "$mysql_p");      mysql_select_db( "$mysql_db");      $sel="select dept_name dept_mast deg_type='???' ";       $val=mysql_query($sel);     $selectbox='<select name=\'userst\'>'; while ($row = mysql_fetch_assoc($val)) { $selectbox.='<option value=\"' . $row['dept_name'] . '\">' . $row['dept_name'] .  '</option>'; } $selectbox.='</select>'; echo $selectbox;         ?> 

how can pass selected value in degree type combobox in place of ???

help me! thanks!

if want values without refreshing whole page, need use ajax. in example, jquery's $.ajax. (simple same page ajax)

<?php  if(isset($_post['get_values'])) {     $data = array();     $selected = $_post['value'];     $host = "localhost";     $mysql_db = "db";     $mysql_u = "root";     $mysql_p = "";     $con = new mysqli($host, $mysql_u, $mysql_p, $mysql_db);     $stmt = $con->prepare('select dept_name dept_mast deg_type = ?');     $stmt->bind_param('s', $selected);     $stmt->execute();     $result = $stmt->get_result();     while ($row = $result->fetch_assoc()){         $data[] = $row;     }      // return value json     echo json_encode($result);     exit; }  ?>  <select id="select3" name="deg">     <option value="ug">ug</option>     <option value="pg">pg</option> </select> <div id="second_select"></div>  <script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script> <!-- <script src="jquery.min.js"></script> --> <script type="text/javascript"> $(document).ready(function(){      $('#select3').on('change', function(){     // when user selects value         var selected = $(this).val(); // value             // request server         $.ajax({             url: document.url, // same page process             type: 'post',             data: {get_values: true, value: selected},             datatype: 'json',             success: function(response) {                         // response of server, , put inside new select box                 $('#second_select').html('<select name="userst"></select>');                 var options = '';                 $.each(response, function(index, element){                     options += '<option value="'+element.dept_name+'">'+element.dept_name+'</option>';                 });                 $('select[name="userst"]').html(options);             }         });     });   }); </script> 

sidenote: it's not discouraged, avoid! using mysql_* functions, use newer , improved version myslqi or pdo