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') );

wordpress query a post by id

/*
WordPress: Query a specific post by its post type & ID.
While editing the post, refer to the URL to find the Post ID Number.
e.g. http://...wp-admin/post.php?post=[Post ID Number]...

*/

// Query for a Case Study with ID #12345
<?php query_posts('post_type=case_studies&p=12345'); ?>