How to get current post_id in function.php file - wordpress

Question

This is my function in function.php file
function getcity(){
global $wpdb;
if($_POST['state'])
{
$id=$_POST['state'];
$district = get_post_meta(get_the_ID() , 'district', true);
var_dump($district);
$result=$wpdb->get_results("SELECT * FROM districts WHERE state_id='$id'");
foreach($result as $row) {
$district_name = $row-
>district_name;
$district_id = $row->district_id;
echo '<option value="'.$district_id.'">'.$district_name.'</option>';
}
}
}
add_action("wp_ajax_nopriv_getcity", "getcity");
add_action("wp_ajax_getcity", "getcity");
I want to get current post id in this function to display selected dropdown value..

Answer 1

Note that $post or get_queried_object_id() do not work until the first query was fired. So these options are available only at the hook template_redirect and later. But functions.php is included much earlier (before the hook after_setup_theme) so this isn't a solution.
A function that should work pretty much anywhere would be:
$url = 'http://' . $_SERVER[ 'HTTP_HOST' ] . $_SERVER[ 'REQUEST_URI' ];
$current_post_id = url_to_postid( $url );
Here is a overview over hook execution order.
As mentioned in the comments url_to_postid may be heavy in some setups, so if your code is executed after template_redirect you better use one of the solutions below as they are most likely much faster than the snippet above:
global $post;
$id = $post->id;
or
$id = get_queried_object_id();

Answer 2

Here is one my function what never fail.
I don't know why WordPress not made something like this before.
/**
* Get current page ID
* #autor Ivijan-Stefan Stipic
* #version 2.0.0
**/
function get_current_page_ID(){
static $current_page_ID;
if(!$current_page_ID) return $current_page_ID;
global $post, $wp_query, $wpdb;
if(!is_null($wp_query) && isset($wp_query->post) && isset($wp_query->post->ID) && !empty($wp_query->post->ID)){
$current_page_ID = $wp_query->post->ID;
} else if(function_exists('get_the_id') && !empty(get_the_id())){
$current_page_ID = get_the_id();
}else if(!is_null($post) && isset($post->ID) && !empty($post->ID))
$current_page_ID = $post->ID;
else if( (isset($_GET['action']) && sanitize_text_field($_GET['action']) == 'edit') && $post = ((isset($_GET['post']) && is_numeric($_GET['post'])) ? absint($_GET['post']) : false))
$current_page_ID = $post;
else if($p = ((isset($_GET['p']) && is_numeric($_GET['p'])) ? absint($_GET['p']) : false))
$current_page_ID = $p;
else if($page_id = ((isset($_GET['page_id']) && is_numeric($_GET['page_id'])) ? absint($_GET['page_id']) : false))
$current_page_ID = $page_id;
else if(!is_admin() && $wpdb)
{
$actual_link = rtrim($_SERVER['REQUEST_URI'], '/');
$parts = explode('/', $actual_link);
if(!empty($parts))
{
$slug = end($parts);
if(!empty($slug))
{
if($post_id = $wpdb->get_var(
$wpdb->prepare(
"SELECT ID FROM {$wpdb->posts}
WHERE
`post_status` = %s
AND
`post_name` = %s
AND
TRIM(`post_name`) <> ''
LIMIT 1",
'publish',
sanitize_title($slug)
)
))
{
$current_page_ID = absint($post_id);
}
}
}
}
else if(!is_admin() && 'page' == get_option( 'show_on_front' ) && !empty(get_option( 'page_for_posts' ))){
$current_page_ID = get_option( 'page_for_posts' );
}
return $current_page_ID;
}
This function check all possible cases and return ID of the current page or false on the fail.