registration page

<?php
session_start();
$errors = array();
    if(isset($_POST['login'])){
    $username =  $_POST['email'];
    $password = $_POST['firstname'];
    $firstname = $_POST['firstname'];
    $lastname  =  $_POST['lastname'];
    $age = $_POST['age'];
    $address  = $_POST['address'];
    $city = $_POST['city'];
    $email  =  $_POST['email'];

    if(file_exists('../users/users.xml')){
        $xml = simplexml_load_file('../users/users.xml');
    }
    foreach($xml->username as $user) if ($user == $username) $errors[] = 'Username already exists';

    if($username == ''){
        $errors[] = 'Username is blank';
    }

    if($email == ''){
        $errors[] = 'Email is blank';
    }
    if($firstname == ''){
        $errors[] = 'Firstname is blank';
    }
    if($lastname == ''){
        $errors[] = 'lastname is blank';
    }
    if ($age <=15 || $age >=91){
        $errors[] = 'Age must be between 16 and 90';
    }
    if ($address == ''){
        $errors[] = 'Address is blank';
    }
    if ($city == ''){
        $errors[] = 'City is blank';
    }
    if(count($errors) == 0){

        $xml = simplexml_load_file('../users/users.xml');
        $nxml = new SimpleXMLElement($xml->asXML());
        $nxml->addChild('username', $email);
        $nxml->addChild('password', $firstname);
        $nxml->addChild('firstname', $firstname);
        $nxml->addChild('lastname', $lastname);
        $nxml->addChild('age', $age);
        $nxml->addChild('address', $address);
        $nxml->addChild('city', $city);
        $nxml->addChild('email', $email);
        $nxml->asXML('../users/users.xml');
        header('Location: products.php');

    }
}
require_once '../functions/functions1.php';?>

<?php echo render_header('dsPc: Shop Online');?>

<?php echo render_nav();?>

<?php echo render_rego();?>

<?php echo render_footer();?>

rego function

function render_rego()
   {
    $output ="
    <div id='bodywrap'>
            <div id='text2'>
                <div class='pad'>";


$output .="
  <form action='' method=post name='registerform'>
  ";
   if(count($errors) > 0){
            echo '<ul>';
            foreach($errors as $e){
                echo '<li>' . $e . '</li>';
            }
            echo '</ul>';
        }
        $output .="
    <fieldset>
    <legend>
    Please Register you details:
    </legend>
    </fieldset>
    <fieldset>
        <label for='firstname'>First Name:</label>
         <input type='text' name='firstname' size='20' />
    </fieldset>
     <fieldset>
        <label for='lastname'>Last Name:</label>
         <input type='text' name='lastname' size='20' />
    </fieldset>
    <fieldset>
        <label for='age'>Age:</label>
        <input type='text' name='age' size='20' />
    </fieldset>
    <fieldset>
        <label for='address'>Address:</label>
            <input type='text' name='address' size='20' />
    </fieldset>
    <fieldset>
        <label for='city'>City:</label>
            <input type='text' name='city' size='20' />
    </fieldset>
    <fieldset>
        <label for='email'>Email:</label>
            <input type='text' name='email' size='20' />
    </fieldset>
    <p class='submit'>
            <input type='submit' name='login' value='Login' />
    </p>
</form>";
        $output .='
                </div>
            </div>
        </div>
    ';
           return $output;
   }