Trying to theme a specific page using a _phptemplate_variables function but i get any result

hello,

I use this fuction in template.php function _phptemplate_variables($hook, $vars = array()) {
if ($hook == 'page' && isset($_GET['pop']) && $_GET['pop']) {
$vars['template_file'] = 'page-popup';
}
return $vars;
}

to change a specific page but still it displays page.tpl he no longer sees this function and it does not execute.

Thx for help