mysql - display sub-menu of parent menu from parent id php -

<?php    $page= $objpage-> get_page();     $id = isset($_get['id']);    $child= $objpage-> get_child_page($id);   ?>       <ul class="nav nav-tabs">         <li role="presentation" class="dropdown">             <?php        while($row = mysql_fetch_array($page) ) { ?>  <a class="dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-  expanded="false">                 <?php echo $row['title']; ?> <span class="caret"></span>            </a>                   <?php } ?>         </li>     </ul> 

function get_page:

function get_page($id="") {  $sql = "select "; if( !empty($field) ){  $sql .=" title page page_id>1 " ;  } else {  $sql .= " * page ";  }  if( !empty($id) ){  $sql .=" page_id=".$id;  } else {  $sql .=" parent_id= -1";  }  $result = mysql_query($sql) or die( mysql_error() );  if( !empty($id) && $result ){  $value = mysql_fetch_array($result) ;  return $value;  }  return $result;  } 

function get_child_page:

function get_child_page($id){  $sql = "select * page parent_id= $id";  $result = mysql_query($sql);  return $result;  } 

i got parent menu has parent id -1 want display child menu parent id .. solution ??