WordPress Redirect from Custom Template

Question

I need to redirect a custom template page to Homepage when a querystring key has an empty value.

for ex: https://example.com/customtemplatepage/?value=1 the Customtemplatepage is a page set with a custom template customtemplate.php in the root of the theme.

Whenever the querystring key "value" is empty it needs to be redirected to the root ("/" or homepage).

1. I tried to catch that in functions.php with add_action('wp_redirect','function');, but it is too early as global $template; is still empty / the customtemplate.php is not loaded yet
2. When I do it in customtemplate.php it is too late to use wp_redirect(); as the headers are already out there

It's possible to use JS with window.location in the customtemplate.php, but that's not an option as we have to do it server side.

Answer 1

The template_include filter should do the trick.

add_filter('template_include', function ($template) {
  // Get template file.
  $file = basename($template);

  if ($file === 'my-template.php') {
    // Your logic goes here.
    wp_redirect(home_url());
    exit;
  }

  return $template;
});

Out of curiosity, why redirect to the homepage? Is a 404 not meant for handling non-existent content?