Revision

<?php

//function registration_install(){}
//function registration_uninstall(){}

//diplay help content
function registration_help($path, $arg) {
  $output = '';  //declare your output variable
  switch ($path) {
    case "admin/help#registration":
      $output = '<p>'.  t("Get the user data with a simple registration form .") .'</p>';
      break;
  }
  return $output;
} // function registration_help



// Valid permissions for this module
function registration_perm() {
  return array('access registration', 'create registration', 'administer registration');
} // function registration_perm()

function registration_register_form(){
    $form = array();

    $form[name] = array(
    '#type' => 'textfield',
    '#title' => t('Enter your name'),
    '#maxlength' => 30,
    '#required' => TRUE,
    '#size' => 20,
    );

    $form[family] = array(
    '#type' => 'textfield',
    '#title' => t('Enter your family name:'),
    '#maxlength' =>50',
    '#required' => TRUE,
    '#size' => 20,
    );

    $form[id] = array(
    '#type' => 'textfield',
    '#title' => t('Enter your ID number:'),
    '#maxlength' => 8,
    '#required' => TRUE,
    '#size' => '8,
    '#element_validate' => array('numric_validate'),
    '#ahah' => array(
          'event' => 'focusout',
          'path' => 'registration/js_idcheck',
          'wrapper' => 'div_idcheck',
          'method' => 'replace',
          'effect' => 'fade'),
    );

    $form['div_idcheck'] = array(
    '#type' => 'value',
    '#value' => '<div id="div_idcheck"></div>',
    );

    $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('send'),
    );
return $form;
}//registration_form

function registration_register_form_submit($form, &$form_state){
    $id=$form_state[values][id];
    $name_f=$form_state[values][name];
    $family=$form_state[values][family];

    // not necessary tovalidate #required = TRUE fields/
    $sql='select * from {registration} where id = %d';
    $result=db_query($sql, $id);
    if(db_num_rows($result) != 0)
        form_set_error(id , 'Your entered id is already exist', FALSE);

    db_query("INSERT INTO {registration} (name, family, id) VALUES ('%s', %s, '%d')", $name_f,$family,$id);
    if (db_error === FALSE)
        drupal_set_message(t('Your form has been saved.'));
    else
        drupal_set_message(t('some error happen.please retry again.'), 'error');
}


function registration_admin_settings() {

 return 0;
}//registration_admin_settings

function registration_menu() {

  $items = array();

  /*$items['admin/settings/registration'] = array(
    'title' => 'View the successful registration.',
    'description' => 'Show the list of registred forms',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('registration_admin_setting'),
    'access arguments' => array('administer registration settings'),
    'type' => MENU_NORMAL_ITEM,
   );
   * */

  //link for registration form
  $items['registration'] = array(
    'title' => 'Registration form',
    'page callback' => 'drupal_get_form',
        'page arguments' => array('registration_register_form'),
    'access arguments' => array('access registration content'),
    'type' => MENU_NORMAL_ITEM,
  );

  //link for submit validate script
/*   $items['registration/js_submit'] = array(
    'page callback' => 'js_submit',
    'access arguments' => array('access mymodule js'),
    'type' => MENU_CALLBACK,
  );*/

  //link for user id validate script
   $items['registration/js_idcheck'] = array(
    'page callback' => 'js_idcheck',
    'access arguments' => array('access mymodule js'),
    'type' => MENU_CALLBACK,
  );

  return $items;
}//registration_menu

//check id field for duplicate entery in database
function js_idcheck(){
    $id=$form_state[values][id];
    $sql="select * from {registration} where id = $id";

    $result = db_query($sql);
    if(db_num_rows($result) != 0)
        return drupal_json(array('status' => FALSE, 'data' => "This ID is already existing in database."));
    else
        return drupal_json(array('status' => TRUE, 'data' => "This ID is new."));
}