<?php
require_once __DIR__ . '/db_connect.php';

$type = '0';
$username = 'username';
$password = 'pass';
$name = 'john';
$mobile = '9090909090';
$email = 'abc@gmail.com';
$alias = strtr($name, array (' ' => '-'));



 define( '_JEXEC', 1 );
 define('JPATH_BASE', dirname(__FILE__) );
 define( 'DS', DIRECTORY_SEPARATOR );

 require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
 require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );

 $app = JFactory::getApplication('site');
 $app->initialise();



 require_once(JPATH_BASE.DS.'components'.DS.'com_users'.DS.'models'.DS.'registration.php');


 $lang = JFactory::getLanguage();
    $extension = 'com_users';
    $base_dir = JPATH_SITE;
    $language_tag = 'en-GB';
    $reload = true;
    $lang->load($extension, $base_dir, $language_tag, $reload);

 $model = new UsersModelRegistration();
 jimport('joomla.mail.helper');
 jimport('joomla.user.helper');




 $block = '0';
 $sendEmail = '0';
 $activation = '1';
 $data = array( 'username' => $username,
 'name' => $name,
 'email1' => $email,
 'password1' => $password, // First password field
 'password2' => $password, // Confirm password field
 'sendEmail' => $sendEmail,
 'activation' => $activation,
 'mobile' => $mobile,
 'groups' =>array("2","10"));
  $response = $model->register($data);
  echo $data['mobile'];

$mobile = null; // line 60 at root/libraries/joomla/user/user.php

<field
        name="mobile"
        type="text"
        description="COM_USERS_DESIRED_USERNAME"
        filter="mobile"
        label="COM_USERS_REGISTER_USERNAME_LABEL"
        message="COM_USERS_REGISTER_USERNAME_MESSAGE"

        size="30"   />

public function register()

<?php
define('_JEXEC', 1);
define('JPATH_BASE', __DIR__);
define('DS', DIRECTORY_SEPARATOR);

/* Required Files */
require_once(JPATH_BASE . DS . 'includes' . DS . 'defines.php');
require_once(JPATH_BASE . DS . 'includes' . DS . 'framework.php');
$app = JFactory::getApplication('site');
$app->initialise();

require_once(JPATH_BASE . DS . 'components' . DS . 'com_users' . DS . 'models' . DS . 'registration.php');
//not necessary
//require_once(JPATH_BASE.DS.'libraries'.DS.'joomla'.DS.'application'.DS.'component'.DS.'helper.php';

$model = new UsersModelRegistration();
jimport('joomla.mail.helper');
jimport('joomla.user.helper');
$language = JFactory::getLanguage();
$language->load('com_users', JPATH_SITE);
$type       = '0';
$username   = 'username';
$password   = 'pass';
$name       = 'john';
$mobile     = '9090909090';
$email      = 'abc@gmail.com';
$alias      = strtr($name, array(' ' => '-'));
$username   = 'adam';
$name       = 'adam24';
$email      = 'adam@mailinator.com';
$password   = 'adam';
$block      = '0';
$sendEmail  = '0';
$activation = '1';
$data       = array('username'   => $username,
            'name'       => $name,
            'email1'     => $email,
            'password1'  => $password, // First password field
            'password2'  => $password, // Confirm password field
            'sendEmail'  => $sendEmail,
            'activation' => $activation,
            'mobile'     => $mobile,
            'groups'     => array("2", "10"));
$response   = $model->register($data);
echo $data['mobile'];

echo $model->register($data);
if (version_compare(PHP_VERSION, '5.3.1', '<')) {
    die('Your host needs to use PHP 5.3.1 or higher to run this version of Joomla!');
}
?>

<!-- db result -->
<tr>
    <th>id</th>
    <th>name</th>
    <th>username</th>
    <th>email</th>
    <th>password</th>
    <th>block</th>
    <th>sendEmail</th>
    <th>registerDate</th>
    <th>lastvisitDate</th>
    <th>activation</th>
    <th>params</th>
    <th>lastResetTime</th>
    <th>resetCount</th>
    <th>otpKey</th>
    <th>otep</th>
    <th>requireReset</th>
    <th>mobile</th>
</tr>

<tr>
    <td>382</td>
    <td>adam24</td>
    <td>adam</td>
    <td>adam@mailinator.com</td>
    <td>$2y$10$CWsR0Ab9AZNvze2l/2Teiek3mzZ3EqyxnAzuYcv9CTaMCeSud5pza</td>
    <td>1</td>
    <td>0</td>
    <td>2015-10-26 22:16:16</td>
    <td>NULL</td>
    <td>fa37ff46e1be16c6029723c395c0f156</td>
    <td>{}</td>
    <td>NULL</td>
    <td>0</td>
    <td></td>
    <td></td>
    <td>0</td>
    <td>9090909090</td>
</tr>