PHP

wordpress get post id

// Retrieve the ID of the current item in the WordPress Loop.
get_the_ID()

wordpress get post by id

$post   = get_post( 123 ); // Where 123 is the ID
$output =  apply_filters( 'the_content', $post->post_content );

wp get post id by slug

url_to_postid( site_url('the_slug') );

wp get_posts return ids

get_posts(array(
    'fields'          => 'ids', // Only get post IDs
    'posts_per_page'  => -1
));