What is the difference between $post_id and get_the_ID()?

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get_the_ID() returns the ID of the current post by way of get_post. Get post assumes the current post to be the global variable $post unless passed parameters specifying otherwise. That means that get_the_ID() should return the same value as contained in $post->ID. Using get_the_ID() lets you avoid dealing with the global directly, though you are dealing with it indirectly. Using get_the_ID() tends to be less error-prone in theme template files.

$post_id is a variable name used commonly to refer the post ID, but it isn’t a Core variable the way that $post is, though it does get defined a few times in specific contexts:

grep -Rn '$post_id' * |grep global
wp-admin/includes/class-wp-comments-list-table.php:25:          global $post_id;
wp-admin/includes/class-wp-comments-list-table.php:45:          global $post_id, $comment_status, $search, $comment_type;
wp-admin/includes/class-wp-comments-list-table.php:141:         global $post_id, $comment_status, $comment_type;
wp-admin/includes/class-wp-comments-list-table.php:250:         global $post_id;
wp-admin/includes/ajax-actions.php:691: global $wp_list_table, $post_id;
wp-includes/ms-deprecated.php:115:function clear_global_post_cache( $post_id ) {

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