Please help, I am trying to get the value of input field after ajax success, I don't know why is it always undefined?
ajax.js
$(document).on('click','.modify',function(){
var modId = $(this).attr('id');
var event_id = $(this).attr('class').split(' ')[1];
$.ajax({
    cache: false,
    url: '../ajax/paraphernalia/ajax_get_edit.php',
    type: 'post',
    data: { modId: modId, event_id: event_id},
    success:function(data){
        var mod_name = $(data).find('input#hidden_headerName').val();
        alert(mod_name);
        $('#display_modal_edit').html(data);
        $('#judge_name_header').html(mod_name);
    }
});
});
ajax_get_edit.php
session_start();
require ("../../global.php");
if (isset($_POST['modId']) && isset($_POST['event_id'])) {
    $modId = $_POST['modId'];
    $event_id = $_POST['event_id'];
    $output = '';
    $sql = mysql_query("SELECT * FROM tbl_judges WHERE judge_id = ".$modId." AND event_id = ".$event_id."");
    $soc_sql = mysql_fetch_assoc($sql);
    $output .= '<input type="text" value="GetThisvalue" id="hidden_headerName">';
    $output .= '.....';//bunch of codes here        
    $output .= '</div>';
    echo $output;
}