In the wordpress loop section, each posts displayed on the page with defined format along with such criteria decided by administrator. The Loop template tag the_ID can be used to easily display the post’s ID inside the post.

Alternatively, inside the the_content filter, declare $id global and use $id to retrieve the post ID. the_ID template tag can be used in this format:

Many time user requires to get Post ID outside the loop template tag. In such cases, the_ID function can not work. In that case, $post->ID will be used to return the post ID. $post is a global object that holds various information about the posts displayed on the page. If the $post is used inside a function, the $post has to be declared as a global variable. For example::

Works inside of the Loop

// Works inside of the Loop
function function_name() {
global $post;
$thePostID = $post->ID;
}

Works in single post outside of the Loop

// Works in single post outside of the Loop
function function_name() {
global $wp_query;
$thePostID = $wp_query->post->ID;
}