index.php and functions.php

//index.php
<?php

require ('functions.php');
// detect whether or not the form has  been posted
//if( isset($_SERVER['REQUEST_METHOD'] == 'POST')  )
if ( $_SERVER['REQUEST_METHOD'] == 'POST')
{
    $name =  trim($_POST['name']);
    $email =  trim($_POST['email']);

    // did they fill everything out ?
    if( empty ($name) || empty($email) )
    {
        $status = 'Please provide a name and valid email address';
    }
    // if not empty add user
    else {
        add_registered_user($name, $email);
        $status = 'thank you for registering';
    }

}


include ('index.tmpl.php');

//----------------- separate file----------------------------
//functions.php

<?php

function add_registered_user($name, $email)
{
    //creates the file mailing_list.php if it doesnt exist
    //then put name and email to that file and append any other submissions
    file_put_contents('mailing_list.php', "$name: $email\n", FILE_APPEND);
}

// --------------------preserve input values after failed post---------------------------------

function  old($key)
{ // return old value if on POST to the value attribute
     if( !empty($_REQUEST['$key']))
      {
            return htmlspecialchars($_REQUEST['$key']);
      }
        return '';
}