User Profile cutomization error...

I'm customizing the user profile with the help of this code and putting in template.php. But when i login to site then nothing is displaythe blank screen come in front of me. But when i use the code after login and update the template.php user profile customize....then user profile works but other functionalities create problems...Plz help

The code below i'm using in template.php

function phptemplate_user_profile($user, $fields = array())
{

return _phptemplate_callback('user_profile', array('user' => $user, 'fields' => $fields));

}

my user profile page is user_profile.tpl.php