Can't get post ID in functions.php?

Question

I need the current post id in a function , which I have written in functions.php. But I can't get the id. I have tried several method.

Like

get_the_ID(); //returns false 


global $post;
$id = $post->ID; //returns null  

global $wp_query
$id =$wp_query->get_queried_object_id(); //returns 0 

$url = 'http://'.$_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"];
$id = url_to_postid($url); //returns 0 

I am using the latest version of wordpress . What can I do now?

UPDATE: I need post id in the function below.

function em_change_form(){
    $id = get_the_ID();
    if(isset($_GET['reg_typ'])) {
        $reg_type = $_GET['reg_typ'];
        if($reg_type =='vln'){
            update_post_meta($id,'custom_booking_form', 2);
        } elseif ($reg_type == 'rsvp') {
            update_post_meta($id,'custom_booking_form', 1);
        }
    }
}

add_action('init','em_change_form');

Answer 1

The post ID is available after the query has been fired.

The first hook that is safe to get post id is 'template_redirect'.

If you can modify your function to accept a post id as argument, like so:

function em_change_form($id){
    $reg_type = filter_input(INPUT_GET, 'reg_typ', FILTER_SANITIZE_STRING);
    if($reg_type === 'vln'){
      update_post_meta($id,'custom_booking_form', 2);
    } elseif ($reg_type == 'rsvp') {
      update_post_meta($id,'custom_booking_form', 1);
    }
}

You can do:

add_action('template_redirect', function() {
  if (is_single()){
     em_change_form(get_queried_object_id());
  }
});

I've used get_queried_object_id() to obtain current queried post id.

If you absolutely need to call your function on an early hook like 'init', you can use url_to_postid(), and home_url() + add_query_arg() to obtain current url:

add_action('init', function() {
  $url = home_url(add_query_arg(array()));
  $id = url_to_postid($url);
  if ($id) {
     em_change_form($id);
  }
});

Note that second method is less performant because url_to_postid() forces WordPress to parse rewrite rules, so if you can, use the first method.